Subnetting(fun With Subnets)

FUN WITH SUBNETS AND

MORE FUN WITH SUBNET MASKS Clarke Green and Bob Stremel June 2002

Why Use Subnets?  Reduce

size of broadcast domains.  Allow Network Administrators more Flexibility.  It’s just good clean fun!

Remember the IP Address?

 

The “NETWORK” part of the IP is assigned by ARIN… The “HOST” part of the IP is assigned by the network administrator.

To Create a Subnet:  Network

administrator chooses.  Borrow (re-purpose) bits from the HOST.

Bits come from the HOST’s First Octet Class A will borrow from the 2nd Octet. Class B will borrow from the 3rd Octet. Class C will borrow from the 4th Octet.

Before subnetting (Class B):

After subnetting (Class B):

Class C Example

Questions/Steps  How

many subnets needed?  What is the subnet mask?  What is the IP in binary?  Addresses in new subnet?

How many subnets needed for: IP 203.15.82.0 ? (Class A, B, or C?)

# of Subnets and Bits Borrowed  Cannot

borrow < 2 bits  Must leave at least 2 bits

Borrowing bits: Available subnets  Where

n = # of bits borrowed  2n – 2 = total subnets available for use  23 – 2 = 6 available subnets

Borrowing bits: Available hosts/subnet  Where

n = # of bits borrowed from 4th

octet  28-n – 2 = total hosts/subnet available for use  25 – 2 = 30 available hosts/subnet ------------------------------------------------- Borrow from 3rd octet? (16-n)  Borrow from 2nd octet? (24-n)

Determine subnet mask  Three

bits borrowed for Class C  Borrowed | Remaining 111 | 00000 = 8 bits in octet  Higher end values of bits = 128 + 64 + 32 = 224  Mask = 255.255.255.224

The subnets:     

 

000| 001| 010| 011| 100|

(not available - network) (1st available) (2nd available) (3rd available) (4th available)

110| 111|

(last available) (not available – broadcast)

Hosts on 1 available subnet st

203.15.82.__  001|00000

(not available – network)

.32  001|00001 (1st available - gateway) .33  001|00010 (2nd available – 1st host) .34

 001|11110

(last available host) .62

 001|11111 (not available – broadcast) .63

Hosts on 2nd available subnet 203.15.82.__  010|00000

(not available – network)

.64  010|00001 (1st available - gateway) .65  010|00010 (2nd available – 1st host) .66

 010|11110

(last available host) .94

 010|11111 (not available – broadcast) .95

Hosts on last available subnet 203.15.82.__   

 

110|00000 (not available – network) .192 110|00001 (1st available - gateway) .193 110|00010 (2nd available – 1st host) .194

110|11110 (last available host) .222 110|11111 (not available – broadcast) .223

ANDing with subnet mask  Routers

do this to ROUTE to correct subnet address.  YOU DO IT to verify your host addresses.  IT WILL ALWAYS RETURN THE SUBNET ADDRESS…WHEN DONE WITH ANY SUBNET HOST.

ANDing  Convert

IP address and subnet mask to

binary:  IP = 203.15.82.33 

11001011.00001111.01010010.00100001 Subnet Mask= 255.255.255.224 11111111.11111111.11111111.11100000

ANDing demonstrated: H = 11001011.00001111.01010010.00100001 M = 11111111.11111111.11111111.11100000 S = 11001011.00001111.01010010.00100000 H = Host M = Subnet Mask S = Subnet address

Verify subnet address – convert to ‘dotted decimal’ S = 11001011.00001111.01010010.00100000 S = 203.15.82.32 That should equal the subnet address!

Yes we are done! Did you have a good time with Clarke and Bob?

I said “We are done!” Go Away!