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Chapter 19. IP Addresses, Network Masks, and Subnets OfficeConnect VPN Firewall User’s Manual

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billion hosts. Because of their huge size, these networks are used for WANs and

by organizations at the infrastructure level of the Internet, such as your ISP.

Class B networks are smaller but still quite large, each able to hold over 65,000

hosts. There can be up to 16,384 class B networks in existence. A class B

network might be appropriate for a large organization such as a business or

government agency.

Class C networks are the smallest, only able to hold 254 hosts at most, but the

total possible number of class C networks exceeds 2 million (2,097,152 to be

exact). LANs connected to the Internet are usually class C networks.

Some important notes regarding IP addresses:

 The class can be determined

easily from field1:

field1 = 1-126: Class A

field1 = 128-191: Class B

field1 = 192-223: Class C

(field1 values not shown are reserved for special uses)

 A host ID can have any value except all fields set to 0 or all fields set to

255, as those values are reserved for special uses.

19.3 Subnet masks

Definition

mask

A mask looks like a regular IP address, but contains a pattern of

bits that tells what parts of an IP address are the network ID and

what parts are the host ID: bits set to 1 mean "this bit is part of the

network ID" and bits set to 0 mean "this bit is part of the host ID."

Subnet masks are used to define subnets (what you get after dividing a network

into smaller pieces). A subnet's network ID is created by "borrowing" one or

more bits from the host ID portion of the address. The subnet mask identifies

these host ID bits.

For example, consider a class C network 192.168.1. To split this into two

subnets, you would use the subnet mask:

255.255.255.128

It's easier to see what's happening if we write this in binary:

11111111. 11111111. 11111111.10000000

As with any class C address, all of the bits in field1 through field 3 are part of the

network ID, but note how the mask specifies that the first bit in field 4 is also

included. Since this extra bit has only two values (0 and 1), this means there are

two subnets. Each subnet uses the remaining 7 bits in field4 for its host IDs,

which range from 0 to 127 (instead of the usual 0 to 255 for a class C address).

Similarly, to split a class C network into four subnets, the mask is:

255.255.255.192 or 11111111. 11111111. 11111111.11000000

The two extra bits in field4 can have four values (00, 01, 10, 11), so there are

four subnets. Each subnet uses the remaining six bits in field4 for its host IDs,

ranging from 0 to 63.

Note

Sometimes a subnet mask does not specify any additional

network ID bits, and thus no subnets. Such a mask is called a

default subnet mask. These masks are:

Class A: 255.0.0.0

Class B: 255.255.0.0

Class C: 255.255.255.0

These are called default because they are used when a network is

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HP OfficeConnect Firewall Series Manuel d'utilisateur Page 126

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