User`s guide

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TCP/IP Network Basics
IP Addressing
Subnets and Subnet Masks
As noted, an IP address consists of a network portion and a host portion.
A
subnet mask is a number that looks like an IP address that shows IP how
many bits are used for the network portion of the IP address by “masking”
the network portion of the IP address. Every IP address must have a subnet
mask. You can use standard or custom subnet masks.To see how subnet
masks are used, we will first define a
subnet.
What is a Subnet? Large networks can be divided by a System administrator by creating a
number of smaller networks, called
subnets. The larger network is divided
into subnets by using
routers (also called default gateways). All devices on
one side of the router form a separate subnet. Each subnet must have a
unique set of IP addresses that are defined by the
subnet mask for that
network. Each side of the router must have a separate network address.
Standard Subnet
Masks
A subnet mask (like an IP address) is 32 bits long, with several contiguous
bits (all 1s) that represent the network portion of the IP address and the rest
of the bits (all 0s) that represent the host portion of the IP address.
For example, the standard subnet mask for a Class A address is
255.0.0.0
(binary
1111 1111.0000 0000.0000 0000.0000 0000), so the first eight bits
(all 1s) are used for the network portion of the IP address and the remaining
24 bits (all 0s) are used for the host portion of the IP address. This table
shows the standard subnet masks for Class A, Class B, and Class C IP
addresses.
Class Standard Subnet
Mask
Standard Subnet Mask
(Binary Equivalent)
A 255.0.0.0 1111 1111. 0000 0000. 0000 0000. 0000 0000
B 255.255.0.0 1111 1111. 1111 1111. 0000 0000. 0000 0000
C 255.255.255.0 1111 1111. 1111 1111. 1111 1111. 0000 0000