Subnetting

Topics: IP address, Subnetwork, Classless Inter-Domain Routing Pages: 43 (1960 words) Published: June 24, 2015
Subnetting

IP Addressing
• Internet Protocol (IP)
• A unique indentifier for a host, or node, on an
IP network
• 32-bit binary number, usually expressed as 4
“dotted decimal” values.
• Each decimal value represents 8 bits, in the
range of 0 to 255

Example
140.179.220.200
140

Written in binary form:
.179
.220
.200

10001100.10110011.11011100.11001000
We see the address in the decimal form
Your computer sees it in the binary form

Binary Octet:
• An octet is made up of eight “1”s and “0”s,
representing the following values:
128 64
32 16 8
4
2 1
• So the value of 140 (the first octet of our
example) looks like this:
1
0
0
0
1
1
0 0

Binary Octet:

1
0
128 + 0 +

0
0
0+ 0+

1
1
0 0
8 + 4 + 0 + 0= 140

Address Classes
• There are 5 different address classes.
• Only 3 are in commercial use at this time.
• You can determine the class of the address by
looking at the first 4 bits of the IP address:
– Class A begin with 0xxx, or 1 to 126 decimal
– Class B begin with 10xx, or 128 to 191 decimal
– Class C begin with 110x, or 192 to 223 decimal
– Class D begin with 1110, or 224 to 239 decimal
– Class E begin with 1111, or 240 to 254 decimal

Interesting Note:
• IP addresses beginning with 011111111, or
127 decimal, is reserved for loopback and
internal testing on a local machine.
• Try it! You should always be able to ping
127.0.0.1, which points to your machine.

Network vs. Host
• Every IP address has 2 parts:
– 1 identifying the network it resides on
– 1 identifying the host address on the
network
• The class of the address and the subnet mask
determine which part belongs to the network
address and which part belongs to the host
address

IP Address Breakdowns:
• The class of the address determines, by default,
which part is for the network (N) and which part
belongs to the Host (n)
Class A: NNNNNNNN.nnnnnnnn.nnnnnnnn.nnnnnnnn
Class B:
NNNNNNNN.NNNNNNNN.nnnnnnnn.nnnnnnnn

Class C:

NNNNNNNN.NNNNNNNN.NNNNNNNN.nnnnnnnn

140.179.220.200
• Our example is a Class B address
• By default, the Network part of the address is
defined by the first 2 octets: 140.179.x.x
• By default, the Host part of the address is
defined by the last 2 octets: x.x.220.200
*Note that the network part of the address is
also known as the Network Address

Two Reserved Addresses on a
Subnet:
• In order to specify the Network Address of a given
IP address, the Host portion is set to all “0”s:
– 140.179.0.0
• If all the bits in the Host portion are set to “1”s, then this specifies the broadcast address that is
sent to all nodes on the network:
– 140.179.255.255

Subnetting
• Subnetting an IP network can be done for
various reasons including:
Organization
Use of different physical media
Preservation of address space
Security
Control network traffic

Subnet Mask
• Subnet masks are applied to an IP address to
identify the Network portion and the Host
portion of the address.
• Your computer performs a bitwise logical AND
operation between the address and the
subnet mask in order to find the Network
Address or number.
• Confused? Read on!

Default Subnet Masks
Class A - 255.0.0.0
11111111.00000000.00000000.00000000
Class B - 255.255.0.0
11111111.11111111.00000000.00000000
Class C - 255.255.255.0
11111111.11111111.11111111.00000000

Logical Bitwise AND Operation
• Remember our example?
– 140.179.240.200
• It’s a Class B, so the subnet mask is:
– 255.255.0.0
We need to look at this as our computer does so
we can perform the bitwise AND...

Logical Bitwise AND Operation
140.179.220.200
Class B address
255.255.0.0
Subnet Mask
In Binary:
10001100.10110011.11110000.11001000
11111111.11111111.00000000.00000000
10001100.10110011.00000000.00000000
By doing this, the computer has found that our
Network Address is 140.179.0.0

Another Example:
Suppose we have the address of:
What class is it? Class C...

References: • TCP/IP 24 seven: The Essential Resource for
Systems Administrators. Gary Govanus. SYBEX,
Network Press. 1999
• www.learntosubnet.com
• www.ralphb.net/IPSubnet. Ralph Becker. 2000
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