Appendix
A
. IP Addresses, Network Masks, and Subnets
Neobit 1012VA-II ADSL Ethernet Router User’s Guide
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Network classes
The three commonly used network classes are A, B, and C. (There
is also a class D but it has a special use beyond the scope of this
discussion.) These classes have different uses and characteristics.
Class A networks are the Internet's largest networks, each with
room for over 16 million hosts. Up to 126 of these huge networks
can exist, for a total of over 2 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.
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
