Understanding IP Addressing: Everything You Ever Wanted To Know

Subnetting

In 1985, RFC 950 defined a standard procedure to support the subnetting, or division, of a single Class A, B, or C network number into smaller pieces. Subnetting was introduced to overcome some of the problems that parts of the Internet were beginning to experience with the classful two-level addressing hierarchy:

• Internet routing tables were beginning to grow.
• Local administrators had to request another network number from the Internet before a new network could be installed at their site.

Both of these problems were attacked by adding another level of hierarchy to the IP addressing structure. Instead of the classful two-level hierarchy, subnetting supports a three-level hierarchy. Figure 6 illustrates the basic idea of subnetting which is to divide the standard classful host-number field into two parts – the subnet-number and the host-number on that subnet.

Figure 6: Subnet Address Hierarchy

Subnetting attacked the expanding routing table problem by ensuring that the subnet structure of a network is never visible outside of the organization’s private network. The route from the Internet to any subnet of a given IP address is the same, no matter which subnet the destination host is on. This is because all subnets of a given network number use the same network-prefix but different subnet numbers. The routers within the private organization need to differentiate between the individual subnets, but as far as the Internet routers are concerned, all of the subnets in the organization are collected into a single routing table entry. This allows the local administrator to introduce arbitrary complexity into the private network without affecting the size of the Internet’s routing tables.

Subnetting overcame the registered number issue by assigning each organization one (or at most a few) network number(s) from the IPv4 address space. The organization was then free to assign a distinct subnetwork number for each of its internal networks. This allows the organization to deploy additional subnets without needing to obtain a new network number from the Internet.

Figure 7: Subnetting Reduces the Routing Requirements of the Internet

In Figure 7, a site with several logical networks uses subnet addressing to cover them with a single /16 (Class B) network address. The router accepts all traffic from the Internet addressed to network 130.5.0.0, and forwards traffic to the interior subnetworks based on the third octet of the classful address. The deployment of subnetting within the private network provides several benefits:

• The size of the global Internet routing table does not grow because the site administrator does not need to obtain additional address space and the routing advertisements for all of the subnets are combined into a single routing table entry.
• The local administrator has the flexibility to deploy additional subnets without obtaining a new network number from the Internet.
• Route flapping (i.e., the rapid changing of routes) within the private network does not affect the Internet routing table since Internet routers do not know about the reachability of the individual subnets – they just know about the reachability of the parent network number.

Extended-Network-Prefix
Internet routers use only the network-prefix of the destination address to route traffic to a subnetted environment. Routers within the subnetted environment use the extended-network- prefix to route traffic between the individual subnets. The extended-network-prefix is composed of the classful network-prefix and the subnet-number.

Figure 8: Extended-Network-Prefix

The extended-network-prefix has traditionally been identified by the subnet mask. For example, if you have the /16 address of 130.5.0.0 and you want to use the entire third octet to represent the subnet-number, you need to specify a subnet mask of 255.255.255.0. The bits in the subnet mask and the Internet address have a one-to-one correspondence. The bits of the subnet mask are set to 1 if the system examining the address should treat the corresponding bit in the IP address as part of the extended-network- prefix. The bits in the mask are set to 0 if the system should treat the bit as part of the host-number. This is illustrated if Figure 9.

Figure 9: Subnet Mask

The standards describing modern routing protocols often refer to the extended-network-prefix- length rather than the subnet mask. The prefix length is equal to the number of contiguous one-bits in the traditional subnet mask. This means that specifying the network address 130.5.5.25 with a subnet mask of 255.255.255.0 can also be expressed as 130.5.5.25/24. The / notation is more compact and easier to understand than writing out the mask in its traditional dotted-decimal format. This is illustrated in Figure 10.

Figure 10: Extended-Network-Prefix Length

However, it is important to note that modern routing protocols still carry the subnet mask. There are no Internet standard routing protocols that have a one-byte field in their header that contains the number of bits in the extended-network prefix. Rather, each routing protocol is still required to carry the complete four-octet subnet mask.

Subnet Design Considerations
The deployment of an addressing plan requires careful thought on the part of the network administrator. There are four key questions that must be answered before any design should be undertaken:
1) How many total subnets does the organization need today?
2) How many total subnets will the organization need in the future?
3) How many hosts are there on the organization’s largest subnet today?
4) How many hosts will there be on the organization’s largest subnet in the future?

The first step in the planning process is to take the maximum number of subnets required and round up to the nearest power of two. For example, if a organization needs 9 subnets, 2 3 (or will not provide enough subnet addressing space, so the network administrator will need to round up to 2 4 (or 16). When performing this assessment, it is critical that the network administrator always allow adequate room for future growth. For example, if 14 subnets are required today, then 16 subnets might not be enough in two years when the 17th subnet needs to be deployed. In this case, it might be wise to allow for more growth and select 2 5 (or 32) as the maximum number of subnets.

The second step is to make sure that there are enough host addresses for the organization’s largest subnet. If the largest subnet needs to support 50 host addresses today, 2 5 (or 32) will not provide enough host address space so the network administrator will need to round up to 2 6 (or 64).

The final step is to make sure that the organization’s address allocation provides enough bits to deploy the required subnet addressing plan. For example, if the organization has a single /16, it could easily deploy 4-bits for the subnet-number and 6-bits for the host number. However, if the organization has several /24s and it needs to deploy 9 subnets, it may be required to subnet each of its /24s into four subnets (using 2 bits) and then build the internet by combining the subnets of 3 different /24 network numbers. An alternative solution, would be to deploy network numbers from the private address space (RFC 1918) for internal connectivity and use a Network Address Translator (NAT) to provide external Internet access.

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