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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
Switch Management and Operating Concepts 83
Each digit in an 8-bit binary number (an octet) represents a
power of two. The left-most digit represents 2 raised to the 7
th
power (2x2x2x2x2x2x2=128) while the right-most digit
represents 2 raised to the 0
th
power (any number raised to the
0
th
power is equal to one, by definition).
IP addresses actually consist of two parts, one identifying the
network and one identifying the destination (node) within the
network.
The IP address discussed above is one part and a second
number called the Subnet mask is the other part. To make
this a bit more confusing, the subnet mask has the same
numerical form as an IP address.
Address Classes
Address classes refer to the range of numbers in the subnet
mask. Grouping the subnet masks into classes makes the task
of dividing a network into subnets a bit easier.
There are 5 address classes. The first 4 bits in the IP address
determine which class the IP address falls in.
• Class A addresses begin with 0xxx, or 1 to 126 decimal.
• Class B addresses begin with 10xx, or 128 to 191 decimal.
• Class C addresses begin with 110x, or 192 to 223 decimal.
• Class D addresses begin with 1110, or 224 to 239 decimal.
• Class E addresses begin with 1111, or 240 to 254 decimal.
Addresses beginning with 01111111, or 127 decimal, are
reserved. They are used for internal testing on a local machine
(called loopback). The address 127.0.0.1 can always be pinged
from a local node because it forms a loopback and points back
to the same node.
Class D addresses are reserved for multicasting.