The TCP/IP Guide - IP Subnet Masks, Notation and Subnet Calculations

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The TCP/IP Guide

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 Subnetting: "Three-Level" Hierarchical IP Subnet Addressing

IP Default Subnet Masks For Address Classes A, B and C

IP Subnet Masks, Notation and Subnet Calculations
(Page 3 of 4)

Applying the Subnet Mask: An Example

Now, let's see how the subnet mask might be used. Suppose we have a host on this network with an IP of 154.71.150.42. A router needs to figure out which subnet this address is on. This is done by performing the masking operation shown in Table 50 and Figure 67.

**Table 50: Determining the Subnet ID of an IP Address Through Subnet Masking**
Component	Octet 1	Octet 2	Octet 3	Octet 4
IP Address	10011010 (154)	01000111 (71)	10010110 (150)	00101010 (42)
Subnet Mask	11111111 (255)	11111111 (255)	11111000 (248)	00000000 (0)
Result of AND Masking	10011010 (154)	01000111 (71)	10010000 (144)	00000000 (0)

**Figure 67: Determining the Subnet ID of an IP Address Through Subnet Masking**
Subnet masking involves performing a boolean *AND* between each corresponding bit in the subnet mask and the IP address. The subnet mask can be likened to a physical mask; each 1 in it lets the corresponding bit of the IP address “show through”, while each 0 blocks the corresponding IP address bit. In this way the host ID bits of the address are stripped so the device can determine the subnet to which the address belongs.

This result, 154.71.144.0, is the IP address of the subnet to which 154.71.150.42 belongs. There is no need to explicitly differentiate the network ID bits from the subnet ID bits, because we are still using “classful” addresses. Any router can see that since the first two bits of the address are “10”, this is a Class B address. So the network ID is 16 bits, and this means the subnet ID must be bits 17 to 21, counting from the left. Here, the subnet is the portion highlighted above: “10010”, or subnet #18. (I'll explain this better in the section on custom subnetting.)

Key Concept: The subnet mask is often expressed in dotted decimal notation for convenience, but is used by computers as a binary number, and usually must be expressed in binary to understand how the mask works and the number of subnet ID bits it represents.

IP Subnetting: "Three-Level" Hierarchical IP Subnet Addressing

IP Default Subnet Masks For Address Classes A, B and C

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Version 3.0 - Version Date: September 20, 2005

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