What is Supernetting?

Supernetting is the process of aggregation of multiple networks into a single network, and it is an inverse process of subnetting. CIDR Value, Network ID, Subnet Mask, Broadcast ID, and Block Size of the route ID are the information required for performing the Supernetting. Supernetting can only be performed on those networks that satisfy the three rules that are explained in this article.

Introduction

Supernetting is the procedure of combining multiple subnetworks into one network. The process of Supernetting is just the inverse of subnetting. In the process of Supernetting, bits of the network are converted into bits of the host. Other names for Supernetting are aggregation and route summarization. This process enables the creation of more host addresses, and these host addresses are created at the expense of the network addresses. Supernetting is performed by the Internet service provider to achieve the most efficient allocation of the IP address. If we see Supernetting, more specifically then,

• Supernetting is defined as combining many networks for the creation of a bigger network
• In route aggregation, Supernetting is used for reducing the routing table updates and routing table size.

Supernetting Components

CIDR Value, Network ID, Subnet Mask, Broadcast ID, and Block Size of every route is required for performing the Supernetting.

• For checking the route alignment, network ID and broadcast ID is required, and for performing, routes must be required to be sequential.
• Block Size is required for the calculation of the summarized route from the routes that are given.
• CIDR value and subnet mask are different notations of a similar thing, and both are used for finding out the number of network bits in the IP address.

Protocol Requirements

Classless Inter-Domain Routing(CIDR) supported Routing protocols are the requirement of Supernetting. Interior Gateway Routing Protocol, Routing Information Protocol (RIPv1), and Exterior Gateway Protocol can not send the information needed for the Supernetting as these protocols assume classful addressing. Enhanced Interior Gateway Routing Protocol (EIGRP) is a CIDR-supported classless routing protocol. This protocol summarizes only in the routing table, and then it transmits these routes that are summarized to its peers. CIDR-supported Other routing protocols may include Open Shortest Path First, EIGRP, RIPv2, IS-IS, and Border Gateway Protocol`.

How to Supernet a Network?

All the networks are not considered suitable networks for aggregation. Some rules are defined for Supernetting the network. The network must follow below given three rules for aggregation.

• Contiguous: All the networks that are to be aggregated must be contiguous.
• Same Size: All the networks must have the same size in the power of 2.
• Divisibility: The ID of the first network must be zero or must be divisible by the block size.

Now let us understand the rules of aggregation by taking an example. Suppose four networks are having the network ID 201.1.0.0, 201.1.1.0, 201.1.2.0, and 201.1.3.0. Now let us check if these networks can be aggregated or not.

• Rule 1: Contiguous- As we can identify from IP addresses these are class C networks. The first network range is from 201.1.0.0 To 201.1.0.255. The second network range starts from 201.1.2.0. The starting address of the second network is obtained by adding 1 to the last network starting IP address. In this way, we will check all network IP addresses, and that all are contiguous.
• Rule 2: Same Size- As all the given IP addresses belong to class C and every network has 2⁸ =256 hosts.
• Rule 3: Divisibility- IP address of the first network must be divisible by the network’s total size. The total size of the network in our example is 4∗2⁸=2¹⁰. When we divide the starting IP address with the network size, then we get the last 10 bits as a remainder. For making the IP address divisible by the size, the last ten digits must be zero. A binary representation of the first IP address of the above example is given below: 11001001.00000001.00000000. 00000000

Here last ten bits are zero. So it can be divisible by the size of the network. So all three conditions are satisfied for the IP addresses given in the example. These four networks can be combined to form a supernet. The supernet ID or the network ID for all four networks will be 201.1.0.0.

Why is Supernetting Done?

To reduce the size of the routing table on the router, we use Supernetting. Let’s take an example in which a router has 8 separate routes (each separate route pointing to the same next hop), instead of a separate route, it can also have the aggregated routes for all these 8 separate routes. The following are the reasons that make aggregated routes important.

• On routing devices, it saves processing resources as well as routing devices. Or we can say that it does not require more space for storing their routing table and also needs limited processing power for searching through the routing table.
• Another important reason is that it provides the facility of stability on the network. If there is any fluctuation in any part of the network, it will not propagate to all the parts of the network, fluctuation can be isolated easily.

Apart from providing the above facilities to the routing tables, it is also beneficial in slowing down the exhaustion of IP addresses by using the Classless Inter-Domain Routing (CIDR). When there is a requirement for more than one class C network, then in replace of providing a class B network, these networks of class C can be aggregated efficiently by using the prefixes of variable length (e.g. /21, /19, etc.). So, it can also be used for increasing the number of addresses available on the network. Let’s take an example, you can aggregate four /24 networks (each having 254 usable IP addresses) for designing one /22 network ( having 1022 usable IP addresses).

Supernetting Rules

Same as subnetting, Supernetting also uses a power of 2 for counting, i.e., 2, 4, 8,16, and so on. While designing a supernet, make sure that it only covers the network you want to aggregate and not more than your requirement. It is even better to use less than required so that the problem of routing tables can be neglected. Following are the rules for designing a supernet:

• It is important to make sure that the network must be contiguous.
• Find out the number of networks you need to aggregate. And this number must be in the power of 2.
• The value of the first non-common octet present in the first IP address block in the list of various networks that need to be aggregated must be compared with the aggregated number of networks( it must be in the power of 2). The following must be the value of the first non-common octet:

1. It must be Zero(0) or
2. It must be the multiple of the aggregated number of networks. Let’s take an example 16 is a multiple of 8 on the other hand, 8 is not the multiple of 16.

For a detailed explanation of the above rules, let’s consider some examples. The following list of networks needs to be aggregated:

Following rules must be applied to each list, and if any list does not follow the rules given, then we are unable to form one supernet for all these particular networks without creating any problems in the network.

Rule 1: Contiguous networks

• Networks given in the list1 of the above table are the contiguous networks as the next IP address after 192.168.0.0/24 is 192.168.1.0/24. So list1 networks can proceed further for Supernetting.
• Networks given in the list2 of the above table are the contiguous networks, as the next IP address after 192.168.1.0/24 is 192.168.2.0/24. So list2 networks can proceed further for Supernetting.
• Networks given in the list3 of the above table are the contiguous networks as the next IP address after 192.168.1.0/24 is 192.168.2.0/24. So list3 networks can proceed further for Supernetting.
• Networks given in the list4 of the above table are not contiguous networks as the next IP address after 192.168.2.0/24 is 192.168.4.0/24 instead of 192.168.3.0/24. So list4 networks can not proceed further for Supernetting.
• Similarly, list5 IP addresses in the above table are also contiguous networks. So list5 networks can also proceed further for Supernetting.

Rule 2: Number of networks order of

• Two networks are given in the list1 of the above table and an order of 2. So list1 networks can proceed further for Supernetting.
• Two networks are given in the list2 of the above table and an order of 2. So list2 networks can proceed further for Supernetting.
• Three networks are given in the list3 of the above table and which are not in an order of 2. So list3 networks can not proceed further for Supernetting.
• Two networks are given in the list5 of the above table and an order of 2. So list5 networks can also proceed further for Supernetting.

Rule 3: The value of the non-common octet in the first IP block is zero or a multiple of the number of networks to be aggregated

• In the IP addresses of list1 first non-common octet is the 3rd octet i.e. 0 vs. 1 and the lowest or first address block is 192.168.0.0/24. 0 is the 3rd octet’s decimal value in this block, and the decimal value is 0. So list1 networks can proceed further for Supernetting.
• In the IP addresses of list 2 first non-common octet is the 3rd octet i.e. 1 vs. 2, and the lowest or first address block is 192.168.1.0/24. 1 is the 3rd octet’s decimal value in this block. The number of networks to be aggregated is 4, and the decimal value is neither zero nor the multiple of 4. So list2 networks can not proceed further for Supernetting.
• In the IP addresses of the list5 first non-common octet is the 2nd octet i.e. 4 vs. 5 vs. 6 vs. 7 and the lowest or first address block is 10.4.0.0/16. 4 is the 2nd octet’s decimal value in this block. The number of networks to be aggregated is 4, and the decimal value is a multiple of 4. So list5 networks can proceed further for Supernetting.

So, networks of List1 and List5 satisfy all three rules and can be aggregated as a single network to form a supernet.

Supernet Mask

A supernet mask is generally a 32-bit number in which 1 is used for the identification of all the fixed bits present in the network whereas 0 is used to represent all the variable bits of the network. So, after subnetting, there are 252 usable ports. And there is also a loss in the number of IP addresses in the network because of Supernetting.

Refer to the below image for the supernet mask

The bits that are present on the left of the red line are fixed bits and the bits that are present on the right of the red line are variable. In the previous example, the routing table present at router 2 is now reduced, and now for all four networks, it has only one entry. But router 1 required a routing table that had four entries for all four networks as it needed to know where to transmit the next packet. The router 2 routing table is given below:

Supernetting Chart

The formula for finding the size of the block is given below: Number of host bits (n) = 32 – CIDR Value Block size = 2ⁿ Below is the list given for the block size and subnet mask of the CIDR values

Examples of Supernetting

Now let us understand the concept of Supernetting with the help of two examples: Example1:

For every route, write Subnet Mask, CIDR value, Broadcast ID, Network ID and block size.

Now we check whether the route ID is sequential or not. ID is sequential if the starting address of the next IP address is from the broadcast ending address of the previous route.

Refer to the below image for checking whether route IDs are sequential or not

Now sum the block sizes of the above example i.e. 256. Now we are required to find the block size of the nearest valid block that provides equal or fewer addresses. 256 block size exactly meets our requirements. Block size 256 is given by the subnetting of the /24. For writing the summarized route, write the first route network ID with the summarized route subnet mask and CIDR value.

In our example1, the first route network ID is 192.168.1.0, and the summarized route CIDR value is /24. So, 192.168.1.0/24 is the summarized route of the above example.

Example2:

For every route write Subnet Mask, CIDR value, Broadcast ID, Network ID, and block size.

Now we check whether the route ID is sequential or not. ID is sequential if the starting address of the next IP address is from the broadcast ending address of the previous route.

Refer to the below image to check whether route IDs are sequential or not

Now sum the block sizes of the above example i.e. 1024. Now we are required to find the block size of the nearest valid block that provides equal or fewer addresses. 1024 block size exactly meets our requirements. The block size of 1024 is given by the subnetting of the /22. For writing the summarized route, write the first route network ID with the summarized route subnet mask and CIDR value.

In our example2, the first route network ID is 10.0.0.0 and the summarized route CIDR value is /22. So, 10.0.0.0/22 is the summarized route of the above example.

Advantages and Disadvantages of Supernetting

Advantages

• Supernetting reduces and controls the traffic on the network
• It also helps in solving the lack of IP address problems.
• Routing table is minimized by the Supernetting.
• By Supernetting, entries of routing information are summarized into a single entry of the routing table which results in a decrement in the size of the routing table; hence Supernetting saves memory space.

Disadvantages

• It is unable to cover different areas of the network when several entries are merged.
• Another disadvantage is that all networks must be in the same class, and every IP address needs to be contiguous.
• In Supernetting, block combinations must be in the power of 2 periodically. Suppose there is a requirement of 3 blocks, then 4 number blocks will be assigned.

Conclusion

• Supernetting is performed for aggregating multiple networks into a single network.
• CIDR Value, Network ID, Subnet Mask, Broadcast ID, and Block Size of every route are required for performing the Supernetting.
• Classless Inter-Domain Routing(CIDR) supported Routing protocols are the requirement of Supernetting.
• Contiguous, Same Size, and Divisibility are the three rules that must be satisfied for performing the Supernetting.
• One reason for doing Supernetting is to reduce the routing table size, thus saving memory.
• For Supernetting, all the networks are to be required in the same class.