Subnet Masks - ZyXEL Communications Dimension ES-2008 User Manual

Similarly the first octet of a class "B" must begin with "10", therefore the first octet of a
class "B" address has a valid range of 128 to 191. The first octet of a class "C" address
begins with "110", and therefore has a range of 192 to 223.
CLASS ALLOWED RANGE OF FIRST
Class A
Class B
Class C
Class D

Subnet Masks

A subnet mask is used to determine which bits are part of the network number, and which
bits are part of the host ID (using a logical AND operation). A subnet mask has 32 bits;
each bit of the mask corresponds to a bit of the IP address. If a bit in the subnet mask is a
"1" then the corresponding bit in the IP address is part of the network number. If a bit in
the subnet mask is "0" then the corresponding bit in the IP address is part of the host ID.
Subnet masks are expressed in dotted decimal notation just as IP addresses are. The
"natural" masks for class A, B and C IP addresses are as follows.
Subnetting
With subnetting, the class arrangement of an IP address is ignored. For example, a class C
address no longer has to have 24 bits of network number and 8 bits of host ID. With
subnetting, some of the host ID bits are converted into network number bits. By
convention, subnet masks always consist of a continuous sequence of ones beginning
from the left most bit of the mask, followed by a continuous sequence of zeros, for a total
number of 32 bits.
Since the mask is always a continuous number of ones beginning from the left, followed
by a continuous number of zeros for the remainder of the 32 bit mask, you can simply
specify the number of ones instead of writing the value of each octet. This is usually
specified by writing a "/" followed by the number of bits in the mask after the address.
IP Subnetting
Chart 2 Allowed IP Address Range By Class
OCTET (BINARY)
00000000 to 01111111
10000000 to 10111111
11000000 to 11011111
11100000 to 11101111
Chart 3 "Natural" Masks
CLASS
A
B
C
Dimension ES-2008 Ethernet Switch
ALLOWED RANGE OF FIRST
OCTET (DECIMAL)
0 to 127
128 to 191
192 to 223
224 to 239
NATURAL MASK
255.0.0.0
255.255.0.0
255.255.255.0
M