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Subnets and Subnet Masks

The Internet Address Classes accommodate three scales of IP internetworks, where the 32-bits of the IP address are apportioned between network IDs and host IDs depending on how many networks and hosts per network are needed. However, consider the class A network ID, which has the possibility of over 16 million hosts on the same network. All the hosts on the same physical network bounded by IP routers share the same broadcast traffic; they are in the same broadcast domain. It is not practical to have 16 million nodes in the same broadcast domain. The result is that most of the 16 million host addresses are unassignable and are wasted. Even a class B network with 65 thousand hosts is impractical.

In an effort to create smaller broadcast domains and to better utilize the bits in the host ID, an IP network can be subdivided into smaller networks, each bounded by an IP router and assigned a new subnetted network ID, which is a subset of the original class-based network ID.

This creates subnets , subdivisions of an IP network each with their own unique subnetted network ID. Subnetted network IDs are created by using bits from the host ID portion of the original class-based network ID.

Consider the example in Figure 1.7. The class B network of 139.12.0.0 can have up to 65,534 nodes. This is far too many nodes, and in fact the current network is becoming saturated with broadcast traffic. The subnetting of network 139.12.0.0 should be done in such a way so that it does not impact nor require the reconfiguration of the rest of the IP internetwork.

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Figure 1.7 Network 139.12.0.0 Before Subnetting

Network 139.12.0.0 is subnetted by utilizing the first 8 host bits (the third octet) for the new subnetted network ID. When 139.12.0.0 is subnetted, as shown in Figure 1.8, separate networks with their own subnetted network IDs (139.12.1.0, 139.12.2.0, 139.12.3.0) are created. The router is aware of the separate subnetted networks IDs and routes IP packets to the appropriate subnet.

Note that the rest of the IP internetwork still regards all the nodes on the three subnets as being on network 139.12.0.0. The other routers in the IP internetwork are unaware of the subnetting being done on network 139.12.0.0 and therefore require no reconfiguration.

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Figure 1.8 Network 139.12.0.0 After Subnetting

A key element of subnetting is still missing. How does the router who is subdividing network 139.12.0.0 know how the network is being subdivided and which subnets are available on which router interfaces? To give the IP nodes this new level of awareness, they must be told exactly how to discern the new subnetted network ID regardless of Internet Address Classes. A subnet mask is used to tell an IP node how to extract a class-based or subnetted network ID.

Subnet Masks

With the advent of subnetting, one can no longer rely on the definition of the IP address classes to determine the network ID in the IP address. A new value is needed to define which part of the IP address is the network ID and which part is the host ID regardless of whether class-based or subnetted network IDs are being used.

RFC 950 defines the use of a subnet mask (also referred to as an address mask) as a 32-bit value that is used to distinguish the network ID from the host ID in an arbitrary IP address. The bits of the subnet mask are defined as follows:

  • All bits that correspond to the network ID are set to 1.

  • All bits that correspond to the host ID are set to 0.

Each host on a TCP/IP network requires a subnet mask even on a single segment network. Either a default subnet mask, which is used when using class-based network IDs, or a custom subnet mask, which is used when subnetting or supernetting, is configured on each TCP/IP node.

Dotted Decimal Representation of Subnet Masks

Subnet masks are frequently expressed in dotted decimal notation. After the bits are set for the network ID and host ID portion, the resulting 32-bit number is converted to dotted decimal notation. Note that even though expressed in dotted decimal notation, a subnet mask is not an IP address.

A default subnet mask is based on the IP address classes and is used on TCP/IP networks that are not divided into subnets. Table 1.14 lists the default subnet masks using the dotted decimal notation for the subnet mask.

Table 1.14 Default Subnet Masks (Dotted Decimal Notation)

Address Class

Bits for Subnet Mask

Subnet Mask

Class A

11111111 00000000 00000000 00000000

255.0.0.0

Class B

11111111 11111111 00000000 00000000

255.255.0.0

Class C

11111111 11111111 11111111 00000000

255.255.255.0

Custom subnet masks are those that differ from these default subnet masks when you are doing subnetting or supernetting. For example, 138.96.58.0 is an 8-bit subnetted class B network ID. Eight bits of the class-based host ID are being used to express subnetted network IDs. The subnet mask uses a total of 24 bits (255.255.255.0) to define the subnetted network ID. The subnetted network ID and its corresponding subnet mask is then expressed in dotted decimal notation as:

138.96.58.0, 255.255.255.0

Network Prefix Length Representation of Subnet Masks

Because the network ID bits must always be chosen in a contiguous fashion from the high order bits, a shorthand way of expressing a subnet mask is to denote the number of bits that define the network ID as a network prefix using the network prefix notation: /<# of bits>. Table 1.15 lists the default subnet masks using the network prefix notation for the subnet mask.

Table 1.15 Default Subnet Masks (Network Prefix Notation)

Address Class

Bits for Subnet Mask

Network Prefix

Class A

11111111 00000000 00000000 00000000

/8

Class B

11111111 11111111 00000000 00000000

/16

Class C

11111111 11111111 11111111 00000000

/24

For example, the class B network ID 138.96.0.0 with the subnet mask of 255.255.0.0 would be expressed in network prefix notation as 138.96.0.0/16.

As an example of a custom subnet mask, 138.96.58.0 is an 8-bit subnetted class B network ID. The subnet mask uses a total of 24 bits to define the subnetted network ID. The subnetted network ID and its corresponding subnet mask is then expressed in network prefix notation as:

138.96.58.0/24

Network prefix notation is also known as Classless Interdomain Routing (CIDR) notation.

note-icon Note

Because all hosts on the same network must use the same network ID, all hosts on the same network must use the same network ID as defined by the same subnet mask. For example, 138.23.0.0/16 is not the same network ID as 138.23.0.0/24. The network ID 138.23.0.0/16 implies a range of valid host IP addresses from 138.23.0.1 to 138.23.255.254. The network ID 138.23.0.0/24 implies a range of valid host IP addresses from 138.23.0.1 to 138.23.0.254. Clearly, these network IDs do not represent the same range of IP addresses.

Determining the Network ID

To extract the network ID from an arbitrary IP address using an arbitrary subnet mask, IP uses a mathematical operation called a logical AND comparison. In an AND comparison, the result of two items being compared is true only when both items being compared are true; otherwise, the result is false. Applying this principle to bits, the result is 1 when both bits being compared are 1, otherwise the result is 0.

IP performs a logical AND comparison with the 32-bit IP address and the 32-bit subnet mask. This operation is known as a bit-wise logical AND. The result of the bit-wise logical AND of the IP address and the subnet mask is the network ID.

For example, what is the network ID of the IP node 129.56.189.41 with a subnet mask of 255.255.240.0?

To obtain the result, turn both numbers into their binary equivalents and line them up. Then perform the AND operation on each bit and write down the result.

10000001 00111000 10111101 00101001 IP Address

11111111 11111111 11110000 00000000 Subnet Mask

10000001 00111000 10110000 00000000 Network ID

The result of the bit-wise logical AND of the 32 bits of the IP address and the subnet mask is the network ID 129.56.176.0.

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