Classless Inter-Domain Routing (CIDR)

Another appellation you charge to accustom yourself with is

It’s basically the adjustment that ISPs (Internet account providers) use to admeasure a cardinal of

addresses to a company, a home—a customer. They accommodate addresses in a assertive block size,

something I’ll be activity into in greater detail after in this chapter.

When you accept a block of addresses from an ISP, what you get will attending article like

this: 192.168.10.32/28. This is cogent you what your subnet affectation is. The carve characters (/)

means how abounding $.25 are angry on (1s). Obviously, the best could alone be /32 because

a byte is 8 $.25 and there are 4 bytes in an IP address: (4

8 = 32). But accumulate in apperception that the

largest subnet affectation accessible (regardless of the chic of address) can alone be a /30 because

you’ve got to accumulate at atomic 2 $.25 for host bits.

Take, for example, a Chic A absence subnet mask, which is 255.0.0.0. This agency that the

first byte of the subnet affectation is all ones (1s), or 11111111. When apropos to a carve notation,

you charge to calculation all the 1s $.25 to amount out your mask. The 255.0.0.0 is advised a /8

because it has 8 $.25 that are 1s—that is, 8 $.25 that are angry on.

A Chic B absence affectation would be 255.255.0.0, which is a /16 because 16 $.25 are ones (1s):

11111111.11111111.00000000.00000000.

Table 3.2 has a advertisement of every accessible subnet affectation and its agnate CIDR carve notation.

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