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Eight binary bits are called a 'byte' or an 'octet'. An octet can represent any decimal value between '0' (00000000) and '255'
(11111111). IP addresses, represented in decimal form, are four numbers whose value is between '0' to '255'. The total
range of IP addresses are then:
Lowest possible IP address -
Highest possible IP address -
To convert decimal numbers to 8-bit binary numbers (and vice-versa), you can use the following chart:
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
power (2x2x2x2x2x2x2=128) while the right-most digit represents 2 raised to the 0
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 and 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.
Class E Addresses are reserved for future use. They are not used for node addresses.
The part of the IP address that belongs to the network is the part that is 'hidden' by the '1's in the subnet mask. This can
be seen below:
Class A
Class B
Class C
For example, the IP address 10.42.73.210 is a Class A address, so the Network part of the address (called the Network
Address) is the first octet (10.x.x.x). The node part of the address is the last three octets (x.42.73.210).
To specify the network address for a given IP address, the node part is set to all "0"s. In our example, 10.0.0.0 specifies the
network address for 10.42.73.210. When the node part is set to all "1"s, the address specifies a broadcast address. So,
10.255.255.255 is the broadcast address for the network 10.0.0.0.
Binary Octet Digit
Decimal Equivalent
Binary Number
128+64+32+16+8+4+2+1=
255
Table 5-1. Binary to Decimal Conversion
NETWORK.node.node.node
NETWORK.NETWORK.node.node
NETWORK.NETWORK.NETWORK.node
0.0.0.0
255.255.255.255
7
6
5
2
2
2
2
128
64
32
16
1
1
1
20
8-port Gigabit Ethernet Switch User's Guide
4
3
2
1
2
2
2
2
0
8
4
2
1
1
1
1
1
1
power (any number raised to the 0
th
th
th
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