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Subnet masks

With the rapid growth of the Internet and the ever-increasing demand for new addresses, the number of available networks and hosts in the standard address class structure can be expanded by borrowing bits from the host portion and using them to allow for more networks. Under this addressing scheme, called subnetting, separating the network and host requires a special mechanism called a subnet mask. A subnet mask, which contains a binary bit pattern of ones and zeros, is applied to an address to extract the network ID for purposes of determining whether an address is on the local network. If not, the address is switched or routed on.

Determining the proper mask value to assign to router and client IP addresses is sometimes difficult. You are usually pretty safe using 255.255.255.0 for your IPNetRouter gateway's private subnet, especially if you never intend to have more than 254 unique LAN clients on your LAN.
 

Boolean Nightmares

Don't worry; you don't need to relive your past-life algebraic nightmares to pass the CCNA exam. Boolean algebra is a process that applies binary logic to yield binary results. What a relief, huh?

Working with subnet masks, you need only four basic principles of Boolean algebra:

• 1 and 1 = 1

• 1 and 0 = 0

• 0 and 1 = 0

• 0 and 0 = 0

Or in other words, the only way you can get a result of a 1 is to combine 1 and 1. Everything else ends up as 0.
 

Address Class

Subnet Mask

Class A

255.0.0.0

Class B

255.255.0.0

Class C

255.255.255.0

There are default standard subnet masks for Class A, B, and C addresses. Table 1 lists the commonly used subnet masks for each IP address class.

Table 1: Standard IP Class Default Subnet Masks

1. If a destination IP address is 206.175.162.21, you know that it's a Class C address and its binary equivalent is 11001110 10101111 10100010 00010101.

2. The default standard Class C subnet mask is 255.255.255.0 and its binary equivalent is 11111111 11111111 11111111 00000000.

3. When these two binary numbers (the IP address and the subnet mask) are combined using Boolean algebra (anding), the network ID of the destination network is the result:
 

206.175.162.21 11001110 10101111 10100010 00010101

and

255.255.255.0 11111111 11111111 11111111 00000000

yields

11001110 10101111 10100010 00000000

4. The result is the network ID of 206.175.162.0.