FUN WITH SUBNETS ANDMORE FUN WITH SUBNET MASKS

Why Use Subnets?Reduce size of broadcast domains.Allow Network Administrators more Flexibility.Its just good clean fun!

Remember the IP Address?The NETWORK part of the IP is assigned by ARINThe 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 HOSTs First OctetClass 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/StepsHow 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 BorrowedCannot borrow < 2 bitsMust leave at least 2 bits

Borrowing bits: Available subnetsWhere n = # of bits borrowed2n 2 = total subnets available for use23 2 = 6 available subnets

Borrowing bits:Available hosts/subnetWhere n = # of bits borrowed from 4th octet28-n 2 = total hosts/subnet available for use25 2 = 30 available hosts/subnet--------------------------------------------------Borrow from 3rd octet? (16-n)Borrow from 2nd octet? (24-n)

Determine subnet maskThree bits borrowed for Class CBorrowed | Remaining 111 | 00000 = 8 bits in octetHigher end values of bits = 128 + 64 + 32 = 224Mask = 255.255.255.224

The subnets:000|(not available - network)001|(1st available)010|(2nd available)011|(3rd available)100|(4th available)

110|(last available)111|(not available broadcast)

Hosts on 1st available subnet 203.15.82.__ 001|00000 (not available network) .32001|00001 (1st available - gateway) .33001|00010 (2nd available 1st host) .34

001|11110 (last available host) .62001|11111 (not available broadcast) .63

Hosts on 2nd available subnet203.15.82.__010|00000 (not available network) .64010|00001 (1st available - gateway) .65010|00010 (2nd available 1st host) .66

010|11110 (last available host) .94010|11111 (not available broadcast) .95

Hosts on last available subnet203.15.82.__110|00000 (not available network) .192110|00001 (1st available - gateway) .193110|00010 (2nd available 1st host) .194

110|11110 (last available host) .222110|11111 (not available broadcast) .223

ANDing with subnet maskRouters do this to ROUTE to correct subnet address.YOU DO IT to verify your host addresses.IT WILL ALWAYS RETURN THE SUBNET ADDRESSWHEN DONE WITH ANY SUBNET HOST.

ANDingConvert IP address and subnet mask to binary:IP = 203.15.82.3311001011.00001111.01010010.00100001 Subnet Mask= 255.255.255.22411111111.11111111.11111111.11100000

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

Verify subnet address convert to dotted decimalS = 11001011.00001111.01010010.00100000S = 203.15.82.32

That should equal the subnet address!

Yes we are done!

I said We are done! Go Away!