Supernetting, also called Classless Inter-Domain Routing (CIDR), is a way to aggregate multiple Internet addresses of the same class. The original Internet Protocol (IP) defines IP addresses in four major classes of address structure, Classes A through D. Each class allocates one portion of the 32-bit Internet address format to a network address and the remaining portion to the specific host machines within the network. Using supernetting, the network address 192.168.2.0/24 and an adjacent address 192.168.3.0/24 can be merged into 192.168.2.0/23. The "23" at the end of the address says that the first 23 bits are the network part of the address, leaving the remaining nine bits for specific host addresses. Supernetting is most often used to combine Class C network addresses and is the basis for most routing protocols currently used on the Internet.

PURPOSE :

Supernetting was created as a way to solve the problem of routing tables growing beyond the ability of current software and people to manage and to provide a solution to the exhaustion of Class B network address space. Supernetting allows one routing table entry to represent an aggregation of networks much like one area code represents an aggregation of telephone numbers in an area.

The Border Gateway Protocol (BGP), the prevailing exterior (interdomain) gateway protocol and the Open Shortest Path First (OSPF) router protocol both support supernetting, but the older exterior or interdomain protocols, the Exterior Gateway Protocol (EGP) and the Routing Information Protocol (RIP) do not support it.

Supernetting a TCP/IP network address is the opposite of subnetting. Supernetting is also known as CIDR (classless interdomain routing) as defined by RFCs 1517, 1518, 1519, and 1520. In IPv4, CIDR is one way of attempting to manage the shortage of TCP/IP addresses until IPv6 takes over. 

Supernetting in itself does not give you more TCP/IP addresses; however, it provides larger single networks for use. Here's how to implement supernetting on your network or support a supernetted network that you may have inherited.

How supernetting works

Supernetting acts to bridge the gap between a Class C network that is limited to 254 addresses and a Class B network that is too large, with over 65,000 addresses. In this way, it's possible to have a "logical" network that offers the number of hosts that best suits your situation.

Supernetting achieves this by making a single network that has your specified number of hosts and corresponding supernet (like a subnet mask). A supernetted address will look like any other TCP/IP address in dotted decimal format (XXX.XXX.XXX.XXX), but it will have a supernetted subnet mask. This looks like a

normal subnet mask, but the last octet is not 0 (however, the leading octets of the supernet mask are still 255). Supernetted addresses will require a default gateway that needs to be supernetted as well.

Address ranges, or blocks, are important in supernetting. They allow you to identify the valid addresses in a tabular format that helps identify boundaries on networks. There are many tables you can create or find on the Internet to plan your networks when using supernetting. Figure A shows a supernetting chart using an example configuration that we'll examine in this article.

Figure A

Supernetting Class C addresses

This represents part of the CIDR/supernetting chart to help determine which supernet option to choose.

CIDR Block Supernet Mask # of Networks* # of Hosts**

/17 255.255.128.0 128 32766

/18 255.255.192.0 64 16382

/19 255.255.224.0 32 8190

/20 255.255.240.0 16 4094

/21 255.255.248.0 8 2046

/22 255.255.252.0 4 1022

/23 255.255.254.0 2 510

/24 255.255.255.0 1 254

/25 255.255.255.128 Less than 1* 126

/26 255.255.255.192 Less than 1* 62

/27 255.255.255.224

Less than 1* 30

/28 255.255.255.240 Less than 1* 14

/29 255.255.255.248 Less than 1* 6

/30 255.255.255.252 Less than 1* 2

*Number of full Class C networks—256 or more available addresses**Available addresses—network and broadcast addresses excluded

This is a chart of the /17 through the /30 block of Class C supernets. These ranges are scalable, helping you select how many networks and hosts you would like to use. You may notice that /24 CIDR block looks familiar, as that is really not a supernetted network but a subnetted single Class C network with a standard 24-bit subnet.

Calculating supernet addresses

Calculating a supernet address is easy if the approach is organized. Using the chart in Figure A, determine how many hosts you want to have available on your network and reference that against the # of Hosts column to select the best match. Then, once you select the appropriate number of hosts, you can look across the chart and see the corresponding supernet mask. With that, you will need to determine a valid starting network.

This starting network must meet certain criteria: All networks are consecutive from your starting network. The third octet of the first network must be an even number (zero is valid for

certain situations). When combining eight networks (like the example below), the third octet of the

network number must be evenly divisible by eight. Create a table listing the available networks(s), addresses, supernet mask(s),

default gateway(s), and other networking objects to outline the network.

Usage scenario

In this example, we'll need approximately 1,220 IP addresses for a training lab scenario that involves 150 people, each of whom requires two servers, five network-attached, multiport serial devices, and their own laptop. We'll also need extra addresses for a few routers (including one for Internet access) and addresses for the instructors. This example would be a good candidate for using CIDR. I'll use the 192.168.16.0 network for our starting address.

To satisfy the 1,220 TCP/IP addresses for this scenario, we can use many of the different CIDR blocks. We will use eight Class C networks, or CIDR /21, to give us 2,048 possible addresses. The 2,048 possible addresses are calculated by taking eight networks that will have 256 addresses each (8 x 256 = 2048). We have to subtract two for the network and broadcast addresses (as in a subnetted network), giving us 2048 – 2 = 2046 possible addresses. Starting with 192.168.16.0, all "connected" networks must be consecutive in the numbering of the third octet. Table A outlines the networks and available addresses.

Table A

Network Available Addresses Usage Circumstances 192.168.16.0           1-255                  First address not available 192.168.17.0           0-255 All addresses in range available 192.168.18.0           0-255 All addresses in range available 192.168.19.0           0-255 All addresses in range available 192.168.20.0           0-255 All addresses in range available 192.168.21.0           0-255 All addresses in range available 192.168.22.0           0-255 All addresses in range available 192.168.23.0           0-254 Last address not available

Note that certain IP addresses are valid with atypical numbers in the last octet of the address. For example, both 192.168.19.0 and 192.168.22.255 are valid addresses for a client, but they may not be available for use by all clients that connect to this network. This is because certain operating systems may not allow these types of addresses to be assigned as an IP address, since they may view the address as a network or broadcast address and as invalid for use as a client address (based on standard TCP/IP usage).

Specifically, Windows NT and 2000 do not allow the use of the X.X.X.255 or X.X.X.0 IP addresses. (For more information on this, see Microsoft Knowledge Base Article 281579.) Because the available hosts for this range of addresses will exceed our requirements, the loss of these few addresses will not be an issue. 

The resulting networks will start at 192.168.16.0 and increase in single increments up to 192.168.23.0. The supernet mask (functions as a subnet mask for all involved network devices/systems) for these networks will be 255.255.248.0. This same supernet and default gateway will be used for all of the networks on this supernet. Rendering the 255.255.248.0 supernet mask is easy from the chart in Figure A, but we will now prove how this is achieved.

We obtain our example supernet mask by taking the number of Class C networks we would like (eight in our case) and subtracting that from 256. This result is 248. We take this value and place it into the third octet of the mask, making our result 255.255.248.0. If we want to have 256 or more Class C networks, this quick rule will not work. The addresses listed in Table A will all be on the same network. For example, there is no route necessary for host 192.168.17.49 to access 192.168.19.244, or any other hosts in the range. 

Implementing a supernetted network

I set up a supernetted network in a lab that I have access to. While I do not have over 1,000 computers, I did allocate all of my computers and virtual machines to reside on each network of this supernetted network. The supernetted network required no settings beyond the IP address, supernet mask, and default gateway options of the operating systems in question. There were no special routing requirements or hardware necessary to quickly implement this network. The supernetted network was implemented easily, and I was able to perform all network activities as if it were a more typical 24-bit subnet mask (255.255.255.0). Addressing, name resolution, and network-based applications all performed without incident.

You can also use supernetting in a reverse fashion by decreasing the number of hosts per network. This is common in ISP situations where you need only a limited number of addresses on the Internet, and the carrier provides you with a subnet mask of 255.255.255.248, for example. This particular supernet means that you will have six available hosts on the network. In this scenario, the first and last addresses are removed for the network and broadcast addresses, so dividing 256 hosts by 32 gives us eight hosts in 32 networks. Removing the first and last addresses for each network gives us six available hosts per network.

Why would I want to use supernetting?

ISPs frequently use supernetting to allocate IP addresses most effectively. There may be scenarios where you have many LANS, WLANs, or VLANs that might be optimally suited for supernetting to best administer your network needs. Keep in mind that supernetting introduces complexity to network administration that needs thorough planning, testing, documentation, and administrator competence.

Most new routing equipment and current operating systems support CIDR in their implementation of the TCP/IP protocol. However, before a supernetting implementation, it is critical to ensure that all components of your network are supernetting-aware. This includes operating systems, network services, routers, routing protocols (RIP2, for example does not support CIDR), and any network-based services used on your network.

Supernetting/CIDR Introduction

Introduction

Supernetting, also known as Classless InterDomain Routing (CIDR), is an awesome subject. It exists thanks to the wide adoption of the Internet, which lead to the exhaustion of the

available IP Addresses. More specifically, supernetting was invented in 1993 with the purpose of extending the 32 bit IP address lifetime until the adoption of IPv6 was complete.

Supernets are used to combine multiple Class C networks into groups, which the router, in turn, treats as one big network. It offers some awesome advantages .The creation of Supernets is also known as Address Aggregation.

The Big Picture

Consider this realistic example: You work for a large ISP with a few hundred networks to which it provides services like Internet access, e-mail etc. These networks, which basically are your ISP's clients, consist of 254 host IPs each (One full Class C network for each client), and they each have a permanent connection to your headquarters via ISDN (represented by the yellow lines) and from there your ISP has a direct connection to the Internet Backbone.

This diagram shows the example network we're talking about. Our main focus is the two routers the ISP has, Router No.1 and Router No.2, because these will be affected when we supernet the networks.

Routers No.1 & No.2 exchange information with each other and update their tables, which contain the networks they know about. Router 2 connects directly to 10 networks and needs to let Router 1 know about each one of them. Router 1 in turn will also advertise these networks to the Internet Backbone Router so it too will know about these networks.

The above setup requires that Router No.1 and the Internet Backbone Router each have more than 13 separate entries in their routing tables to make sure that each network is accessible from them. This is not so bad for this example, but try to imagine the problems and the complexity of a similar setup where you have thousands of networks, where the routing tables would be enormous ! Also, you should keep in mind that the larger the routing table, the more work the router needs to do because it has a huge table of routes to maintain and look through all the time.

By using Supernetting, we could supernet the whole network so it appears to the Internet as follows:

You can clearly see that all the clients' networks have been combined into one big network. Even though Router No.1 and the Internet Backbone router see only one big network, Router No.2 knows all about the smaller Class C networks since it is the one "hiding" them from the rest of the world and makes sure it sends the correct data to each network.

We are going to look at a more detailed example later on so we can understand exactly how supernetting works.

NOTE

There are some limitations with Supernetting - this is why there is a rule which we must follow so we don't bump into big routing problems and upset the network. We will have a closer look at the rule on the next page.

The reason for evolution

Supernetting has become very popular and there are a lot of reasons why:

Class B network address space has nearly been exhausted

A small percentage of class C network addresses have beenassigned to networks

Routing tables in Internet routers have grown to a size beyond the ability of software and people to effectively manage

The 32-bit IP address space will eventually be exhausted

How Supernets work

If you understand how Subnetting works, then you will surely understand Supernetting.

Supernets are the opposite of Subnets in that they combine multiple Class C networks into blocks rather than dividing them into segments.

When Subnetting, we borrow bits from the Host ID portion, which increases the number of bits used for the Network ID portion. With Supernetting we do exactly the opposite, meaning we take the bits from the Network ID portion and give them to the Host ID portion, as illustrated in the picture below:

The next page deals with a detailed example to give you an in-depth analysis of Supernetting. The main concept you need to understand is that Supernetting is all about combining multiple Class C networks into one or more groups and it does this by taking bits from the Network ID portion and, by doing so, the bits assigned to the Host ID portion increase.

I think that's a pretty good introduction to Supernetting :)

Let's take a look at an example to see and understand how exactly Supernetting works in practice.

 

Next - Supernetting/CIDR Analysis

This diagram shows the example network we're talking about. Our main focus is the two routers the ISP has, Router No.1 and Router No.2, because these will be affected when we supernet the networks.

Routers No.1 & No.2 exchange information with each other and update their tables, which contain the networks they know about. Router 2 connects directly to 10 networks and needs to let Router 1 know about each one of them. Router 1 in turn

will also advertise these networks to the Internet Backbone Router so it too will know about these networks.

The above setup requires that Router No.1 and the Internet Backbone Router each have more than 13 separate entries in their routing tables to make sure that each network is accessible from them. This is not so bad for this example, but try to imagine the problems and the complexity of a similar setup where you have thousands of networks, where the routing tables would be enormous ! Also, you should keep in mind that the larger the routing table, the more work the router needs to do because it has a huge table of routes to maintain and look through all the time.

By using Supernetting, we could supernet the whole network so it appears to the Internet as follows:

You can clearly see that all the clients' networks have been combined into one big network. Even though Router No.1 and the Internet Backbone router see only one big network, Router No.2 knows all about the smaller Class C networks since it is the one "hiding" them from the rest of the world and makes sure it sends the correct data to each network.

We are going to look at a more detailed example later on so we can understand exactly how supernetting works.

The reason for evolution

Supernetting has become very popular and there are a lot of reasons why:

Class B network address space has nearly been exhausted

A small percentage of class C network addresses have beenassigned to networks

Routing tables in Internet routers have grown to a size beyond the ability of software and people to effectively manage

The 32-bit IP address space will eventually be exhausted

How Supernets work

Supernets are the opposite of Subnets in that they combine multiple Class C networks into blocks rather than dividing them into segments.

When Subnetting, we borrow bits from the Host ID portion, which increases the number of bits used for the Network ID portion. With Supernetting we do exactly the opposite, meaning we take the bits from the Network ID portion and give them to the Host ID portion, as illustrated in the picture below:

The main concept to understand is that Supernetting is all about combining multiple Class C networks into one or more groups and it does this by taking bits from the Network ID portion and, by doing so, the bits assigned to the Host ID portion increase.

Let's take a look at an example to see and understand how exactly Supernetting works in practice.

Supernetting/CIDR Analysis

Guideline - Rule to Supernetting / CIDR

Before we get in to deep waters, we must talk about the main rule that applies to creating Supernets. For our example, this rule dictates that, in order to create Supernets from Class C IP Addresses, the network address must be consecutive and the third octec of the first IP Address must be divisible by two.

If we had 8 networks we wanted to combine, then the third octec of the first IP address would need to be divisible by eight and not two.

There is one more rule you should know and this rule has to do with the routers of the network, which will need to work with the new changes. This rule dictates that all

routers on the network must be running static routing or using a classless routing protocol such as RIP2 or OSPF.

Classless routing protocols include the subnet mask information and can also pass supernetting information. Routing protocols such as RIP1 do NOT include subnet mask information and would just create problems!

The Example

Here is an example involving two companies that want to use Supernetting to solve their network requirements. We are going to determine which company meets the criteria for a Supernet (we are assuming the routers are setup in a way that will support supernetting):

As you can see, Companys No.1's network passes the test, therefore we can Supernet its two networks.

The Analysis of Company 1's Network & creation of its Supernet

Let's now take Company No.1's network, see how the Supernet will be created and determine various important parameters like the new network's broadcast address, the identification of the new supernets etc.

To begin, we must take our two networks and look at them in binary format, this is the only way to "see" exactly what we're doing when supernetting, and take a look at the Network and Host ID portions:

Now we need to create the Supernet. This means that we are going to take one bit from the Network ID of these networks and give it to the Host ID portion. This 1 Bit is our Supernet ID. So our subnet mask will now be reduced from 24 bits to 23 bits. You might get confused or ask why we call this extra Bit we are giving to the Host ID a Supernet ID?

The answer is simple, the one Bit that we are taking from the Network ID is given to the Host ID but, in order for us to clearly "see" where the supernet is created, we colour it Green and give it the "Supernet ID" label:

So there you have it, a new supernet created!

Now We have one Supernet made from two networks (203.31.218.0 and 203.31.219.0). In order to identify these two networks we name the first one (203.31.218.0) Supernet 0 and the second one (203.31.219.0) Supernet 1. This is to distinguish between the two networks.

It actually makes more sense if you look at the values the Supernet ID field takes:

It's very important to understand that Supernet 0 and 1 are part of the same new network ! This means that there is only one network address, one network broadcast address and not two as you might expect.

Let's now have a look at some more important information regarding the new network:

ITEM VALUE

Supernet range 203.31.218.0 - 203.31.219.255

Subnet Mask 255.255.254.0

Supernet Network Address 203.31.218.0

Supernet Broadcast Address 203.31.219.255

Supernet 0 203.31.218.0

Supernet 1 203.31.219.0

Valid IP Address range 203.31.218.1 - 203.31.219.254

Reserved IP Addresses 203.31.218.0, 203.31.219.255

The above table shows pretty much all the information someone would need about the new network.

IP Addresses 203.31.218.255 and 203.31.219.0, which would have been used as the broadcast address for our first old network and the network address of our old second network, are now usuable addresses!

Yes, you can actually assign them to hosts, because we have a Supernet. Now, even though you can use these addresses, I would probably not use them unless I really needed to.

Also, every host that will be part of this Supernet will need to be configured with the new Subnet mask, 255.255.254.0 as noted in the table above. Any host that isn't reconfigured will have big problems trying to communicate with the rest of the network.

This completes the analysis of our Supernet example.

The Supernetting/CIDR Chart

The Supernetting/CIDR chart

There are four columns available in our chart:

The CIDR Block, the Supernet Mask, Number of Class C Networks and the Number of Hosts column.

Class C

CIDR Block Supernet Mask Number of Class C Networks Number of Hosts

/14 255.252.0.0 1024 262144

/15 255.254.0.0 512 131072

/16 255.255.0.0 256 65536

/17 255.255.128.0 128 32768

/18 255.255.192.0 64 16384

/19 255.255.224.0 32 8192

/20 255.255.240.0 16 4096

/21 255.255.248.0 8 2048

/22 255.255.252.0 4 1024

/23 255.255.254.0 2 512

/24 255.255.255.0 1 254

/25 255.255.255.128 1/2 126

/26 255.255.255.192 1/4 62

/27 255.255.255.224 1/8 32

/28 255.255.255.240 1/16 16

/29 255.255.255.248 1/32 8

/30 255.255.255.252 1/64 4

The CIDR Block

The CIDR Block simply represents the number of bits used for the subnet mask. For example, /14 means 14 bits assigned to the subnet mask, it is a lot easier telling someone you have a 14 bit subnet mask rather than a subnet mask of 255.252.0.0

Note: In the above paragraph, we called the 14 bits as a subnet mask, when in fact it's a supernet mask, but because when you configure any network device, the field you will need to enter the value is usually named as the 'subnet mask'.

Now pay particular attention to the CIDR Block /24, and /25 to /30. These blocks are highlighted in yellow and blue to grab your attention.

When we use a CIDR Block of 24 (24 bit subnet mask) we are not Supernetting ! This is a default subnet mask for a Class C network. With CIDR Blocks /25 to /30 we are actually Subnetting and not Supernetting !

Now you might wonder why I have them in the chart. The fact is that those particular CIDR Blocks are valid, regardless of whether applying them to a network means we are Subnetting and not Supernetting.

A good example is if you wanted a permanent connection to your ISP and only required 2 IP Addresses, one for your router and one for your Firewall, you would be assigned one /30 CIDR Block. With such a subnet mask you will have 4 IP Addresses, from which 2 will be reserved (one for the

Network address and one for the Broadcast address) and you're left with 2 that you can assign to your hosts (router and firewall).

The Supernet Mask

Basically, this is your Subnet mask. When you configure the devices that will be attached to the specified network, this is the value you will enter as a Subnet mask. It's also the decimal value the CIDR Block specifies. For example, a /24 CIDR block means a 24 bit Subnet mask, which in its turn translates to 255.255.255.0 .

Number of Class C Networks

This number shows us how many Class C Networks are combined by using a specific Supernet mask or, if you like, CIDR Block. For example, the /24 CIDR Block, 255.255.255.0 Supernet mask is 1 Class C Network, whereas a /20 CIDR Block, 255.255.240.0 Supernet mask is 16 Class C networks.

Number Of Hosts

This value represents the number of hosts per Supernet. For example, when we use a /20 CIDR Block, which means a Subnet (or Supernet) mask of 255.255.240.0, we can have up to 4096 hosts. The value 4096 does not represent the valid, usable IP Addresses. If you wanted to find out how many of these IP Addresses you can actually use, in other words, assign to hosts, then you simply take 2 IP Addresses from that number (the first and last IP Address), so you're left with 4094 IP Addresses to play with.

Why take 2 away?-One is reserved for the Network Address and one for the Broadcast Address of that network!

 

3.

Supernetting is the idea of combining two or more blocks of IP addresses that together compose a continuous range of addresses (no missing addresses in the middle). You create a supernet when you have a need to place more hosts on a single network than currently will work in a classful configuration.

The term supernetting stems from the idea that at one time there were 'classes' of IP addresses and that certain classes of addresses were a certain size. By adding two sets of addresses of one size using supernetting, you created a larger set of addresses--a super subnet that contained both of the smaller classful subnets.

Classful addressing identifies a class C addresses as being in the range of 192.0.0.0 through 223.255.255.255. A single class C block such as 192.168.1.0 - 192.168.1.255 has at most, 254 addresses. If you have more than 254 computers that need to be on the same network (to get Windows Domain Browsing working for instance). then you need to create a supernet.

You may recall that every IP address contains a host portion that identifies the individual host and a network portion that identifies which network the host is attached to. Under the classful addressing scheme, no masks are needed. The address itself describes which part of the IP address is the network portion and which part of the address is the host portion.

The idea of supernetting is the idea of borrowing one or more bits from the network

Supernetting a network

Supernetting (also known as route summarization or route aggregation) uses classless interdomain routing (CIDR) to address a number of IP subnets with a single route. We call that single route a summarized route or a supernet (in other words, the inverse of a subnet).

To learn how to supernet a network, let's look at another example. Let's say we have four IP subnets on the four LAN interfaces of our router: 1.1.0.0/24, 1.1.1.0/24, 1.1.2.0/24, and 1.1.3.0/24. We want to summarize these networks into a single route that we can advertise across the WAN, which reduces the number of routes in the remote routers.

We could summarize these routes with this supernet IP address: 1.1.0.0/22. This single IP address references all four of the IP subnets. Here's a look at it in binary form:

IP address: 00000001 00000001 00000000 0000000

Supernet subnet mask: 11111111 11111111 11111100 0000000

Notice the third octet of the supernet subnet mask: 11111100. This allows the last two bits of the third octet to be any combination of 00, 01, 10, or 11. So when advertised, this supernet mask would show that any of the four subnets are available from the router.

Keep in mind that when subnetting or supernetting from the classful boundaries, you must use a routing protocol that supports variable length subnet masks (VLSM) and CIDR. Your options include Routing Information Protocol version 2 (RIPv2), Enhanced Interior Gateway Routing Protocol (EIGRP), the Open Shortest Path First (OSPF) protocol, and Border Gateway Protocol (BGP).

Of these protocols, EIGRP is the only one that summarizes at classful network boundaries by default—a capability that you can turn on or off. On the other hand, OSPF requires manually entering a summary route with the summary-address command. BGP disables autosummary by default, but you can turn it on, or you can use the aggregate-address command to create your own summary route.

Whether you call it route summarization, route aggregation, or supernetting, this practice is essential on the Internet. If every carrier advertised every specific route it has, it would overwhelm the memory of the Internet BGP routers. For example, my company's BGP Internet router has 125,000 routes to Internet networks, and most of these routes are supernets. However, because the advertising routers summarize their routes, the router is able to receive all Internet routes using only 125,000 entries

Definition of: supernetting

Combining several IP network addresses into one IP address. Supernetting reduces the number of entries in a routing table and is done in CIDR addressing as well as internal networks.

In the following example, a group of networks with contiguous numbers starting with 172.16.8.0 and ending with 172.16.16.0 are supernetted into the subnet mask of 255.255.224.0. The subnet mask is derived by comparing the binary of the first and last addresses. The last bit location on the right that is the same in both addresses

marks the end of the mask. The CIDR notation for this is /19, because there are 19 1 bits in the subnet mask. For example, the IP address 172.16.8.1 would be 172.16.8.1/19. See CIDR.

IP Network Binary Representation172.16.8.0 10101100.00010000.00001000.00000000172.16.16.0 10101100.00010000.00010000.00000000||Subnet Mask ||255.255.224.0 11111111.11111111.11100000.00000000