ZyXEL Communications Dimension ES-2008, User Manual

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Dimension ES-2008 Ethernet Switch
IP Subnetting M
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.
Chart 2 Allowed IP Address Range By Class
CLASS ALLOWED RANGE OF FIRST
OCTET (BINARY)
ALLOWED RANGE OF FIRST
OCTET (DECIMAL)
Class A 0 0000000 to 0 1111111 0 to 127
Class B 10 000000 to 10 111111 128 to 191
Class C 110 00000 to 110 11111 192 to 223
Class D 1110 0000 to 1110 1111 224 to 239
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.
Chart 3 “Natural” Masks
CLASS NATURAL MASK
A 255.0.0.0
B 255.255.0.0
C 255.255.255.0
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.