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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP Subnet Addressing ("Subnetting") Concepts

IP Custom Subnet Masks 1 2 IP Subnetting Summary Tables For Class A, Class B and Class C Networks

The main advantage of conventional “classful” addressing without subnets is its simplicity. Even though we've seen that there can be problems with how to manage thousands of devices in a single Class B network, for example, there is little complexity in how to assign addresses within the network. They are all lumped together, so any combination of bits can be used within the host ID (except for all-zeroes and all-ones, which are special).

When we subnet, however, we create a two-level structure within the “classful” host ID—subnet ID and host ID. This means we must choose IP addresses for devices more carefully. In theory, we are selecting subnets to correspond to our physical networks within the organization, so we want to assign IP addresses in a way that is consistent with our physical network structure.

Once we decided how many subnets we are going to have, we need to identify the subnets and determine what the addresses are for the subnets. Let's start by looking at the subnet identifier. This is just the subnet ID of any of the subnets in our network. Subnets are numbered starting with 0 and increasing up to one less than the maximum number of subnets, which is a function of how many bits are in the subnet ID. (If the all-zero and all-one subnet IDs are excluded as specified in RFC 950 then the first subnet identifier is one).

Of course, we may not need all of the subnets that can theoretically be defined. For example, if we have 20 subnets, we need 5 bits for the subnet identifier, which allows a theoretical maximum of 32 subnets. We would use only subnets 0 to 19; 20 through 31 would be reserved for future use. These subnets could be expressed either in decimal form (0, 1, 2 … up to 19) or in binary (00000, 00001, 00010 … up to 10011).

For each subnet we can also determine the subnet address. To do this we start with the IP address for the overall network, which recall has all zeroes in the “classful” host ID field (8 bits, 16 bits or 24 bits). We then insert the subnet identifier for a particular subnet into the designated subnet bits.

As an example, let’s refer back to the subnetting of a Class B network, 154.71.0.0, shown in Figure 66. We used five subnet ID bits there. We start with the following network IP address, with the subnet ID bits highlighted:

10011010 01000111 00000000 00000000

To find the address of say, subnet #11, we would substitute “01011” for these bits, leaving the host ID bits zero:

10011010 01000111 01011000 00000000

We can then convert this from binary form to dotted decimal, resulting in a subnet address of 154.71.88.0.

IP Custom Subnet Masks 1 2 IP Subnetting Summary Tables For Class A, Class B and Class C Networks