IP Addressing and Subnet Masking
IP addresses are 32 bits long and are represented as a quad octet set.
Each octet represents 1 byte (8 bits) and has a range of 0-255 for 256 values (
2 8 = 256 ). IP networks are divided into classes, with each
class having a network ID and host range. Each class uses bits from the
first octet as part of the network ID and depending on the network class can use
up to two other octets for the network ID with the remaining octets reserved for
hosts on the network. For example, class A networks use the first octet as
the network range and the other three octets are for hosts on the networks and
class C networks use the first three octets from the left as the network ID,
leaving only one octet available for hosts. The Network classes are
divided into classes as follows.
| IP Address Classes |
| Class |
First Octet's Range Decimal |
First Octet's Binary Range |
Network
Host |
Default Subnet Mask |
| A |
0-126 |
00000000 - 01111110 |
Network.Host.Host.Host
Example
10.128.22.100 |
255.0.0.0 |
| B |
128-191 |
10000000 - 10111111 |
Network.Network.Host.Host
Example
172.12.15.65 |
255.255.0.0 |
| C |
192-223 |
11000000 - 11011111 |
Network.Network.Network.Host
Example
209.38.1.2 |
255.255.255.0 |
D
E |
224-239
240-255 |
11100000 - 11111111 |
D is for multicast
E is experimental |
N/A |
Some IP addresses are reserved for special use and are not to be used as
networks.
| Reserved IP Addresses |
| Address |
Use |
| 127.0.0.1 |
Used for loopback |
| 0.0.0.0 |
Used as default route on Cisco Equipment |
| 255.255.255.255 |
Broadcast to all nodes on network |
IP Network addressing
- A network address uniquely identifies each network.
- Every machine on the network shares the same network portion of it's IP
address.
- The node portion of the IP address uniquely identifies the node on its
network, can also be called host address.
Identifying the Parts of IP Network Addresses
This table has one example with the other fields left blank for your
practice.
| Address |
Class |
Network Portion |
Host Portion |
Default Subnet |
| 63.125.22.14 |
A |
63. |
125.22.14 |
255.0.0.0 |
| 199.2.2.10 |
|
|
|
|
| 189.88.25.1 |
|
|
|
|
| 16.12.1.84 |
|
|
|
|
| 145.1.40.2 |
|
|
|
|
| 221.220.21.20 |
|
|
|
|
Process For Subnet masking
- How many subnets?
2(masked bits) - 2 = Subnets
- How many valid hosts per subnet?
2(unmasked bits) - 2 = Hosts
- What are the valid subnets?
256-(subnet base)=Base number
- What are the valid hosts in the subnets?
All numbers between subnets minus the all 1s (.255) and all 0s (.0)
host addresses.
- What is broadcast address of the subnet?
All the host bits turned on.
Example of subnet masking
(255.255.255.192) =
(11111111.1111111.1111111.11000000)
- (22)-2 = 2 Subnets
- (26)-2 = 62 Hosts per subnet
- 256 - 192 = 64 (.01000000) {For the first subnet}
- 65 to 126 (.01000001 to .01111110) Valid hosts
in the subnets
- 127 (.01111111) Broadcast
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