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IP subnet basics

 

Ip addresses are 32 bits (4 octets or 4 x 8 = 32)

IP addresses are constructed in classes based on 1st octet number

Each class has different network id and host id fields

 

FIELDS                         1st octet      2nd octet     3rd octet     4th octet

 

Class A      1-127         net              hosts          hosts          hosts

Class B      128-191     net              net              hosts          hosts

Class C      192-223     net              net              net              hosts

 

 

 

 


In a typical office we may have Sales, Warehousing, Accounting, Administration, and others.

We may not want everyone on the same network.

 

TO MANAGE OUR NETWORK WE MAY WANT TO FURTHER SUB-divide our network.

 

We can use the following technique to create manageable sub-networks.

 

We can borrow bits from the host area and create subnets.

We borrow from the left side of the host bits first (Class C address shown)

 

                                                   (Host bits field)

EX;                  192 . 101 . 114 . 00000000              (last octet is in binary form)

 


Borrow 3 bits     -       So 2 raised to 3rd (23) is 8 possible subnet address combinations

 

The binary would look like this:                         My 8 subnet addresses would be:

 

000 00000                 .0                                             192.101.114.0

001                            .32                                           192.101.114.32

010                            .64                                           192.101.114.64

011                            .96                                           192.101.114.96

100                            .128                                        192.101.114.128

101                            .160                                        192.101.114.160

110                            .192                                        192.101.114.192

111 00000                 .224                                        192.101.114.224

 

 

 

 

 

 

 

 

 

The remaining 5 digits in each subnet are possible host positions.    (25)

So 2 to the 5th is 32 potential hosts.  In the first potential subnet they would look like this.

000 00000

000 00001

000 00010

000 00011

000 00100

000 00101

    on to

000 11111

 

 

 

NOTE: The second potential subnet would look just like the first subnet
but the first three digits in the octet would be 001 instead of 000.

 

Now in the original network 192.101.114.0 (without subnetting) we couldn’t use the .0 or the .255 positions as hosts.  That is because the .0 is the network ID and the .255 is the broadcast number for everyone in that IP network. 
            So we have 256 - 2 = 254 actual host positions left.

            After subnetting we cannot use the first subnet or the last subnet for the same reasons.  That’s right - we lose both subnets (first & last) and all their 64 host positions.   (Possible 8 subnets – 2 = 6 usable subnets)

 

            In each of the other subnets, the first number is the subnet ID and the last number is it’s own broadcast number.  So we use two of the 32 possible host positions in each subnet.    (32 hosts – 2 = 30 usable hosts)

 

            We had a total potential of 256-2=254 hosts.  After borrowing 3 bits to subnet, we have only:           (8-2) 6 subnets X (32-2) 30 hosts = 180 actual host positions.

 

 

We pay a price for management, don’t we?


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