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TABLE 3.1

Default Subnet Mask

Class

Format

A

network.node.node.node

B

network.network.node.node

C

network.network.network.node

110

Chapter 3

  • IP Subnetting and Variable Length Subnet Masks (VLSMs)

255.255.255.0

255.255.0.0

Default Subnet Mask

255.0.0.0

Classless Inter-Domain Routing (CIDR)

Another term you need to familiarize yourself with is Classless Inter-Domain Routing (CIDR). It’s basically the method that ISPs (Internet Service Providers) use to allocate an amount of addresses to a company, a home—a customer. They provide addresses in a certain block size, something I’ll be going into in greater detail later in this chapter.

When you receive a block of addresses from an ISP, what you get will look something like this: 192.168.10.32/28. This is telling you what your subnet mask is. The slash notation (/) means how many bits are turned on (1s). Obviously, the maximum could only be /32 because a byte is 8 bits and there are four bytes in an IP address: (4 8 = 32). But keep in mind that the largest subnet mask available (regardless of the class of address) can only be a /30 because you’ve got to keep at least 2 bits for host bits.

Take for example a Class A default subnet mask, which is 255.0.0.0. This means that the first byte of the subnet mask is all ones (1s), or 11111111. When referring to a slash notation, you need to count all the 1s bits to figure out your mask. The 255.0.0.0 is considered a /8 because it has 8 bits that are 1s—that is, 8 bits that are turned on.

A Class B default mask would be 255.255.0.0, which is a /16 because 16 bits are ones (1s): 11111111.11111111.00000000.00000000.

Table 3.2 has a listing of every available subnet mask and its equivalent CIDR slash notation.

Subnet Mask

CIDR Value

255.0.0.0

/8

255.128.0.0

/9

255.192.0.0

/10

255.224.0.0

/11

255.240.0.0

/12

TABLE 3.2

CIDR Values

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