NAD710 - Introduction to Networks Using Linux

Splitting a class C network into 4 subnets

February 2,2001Professor Tom Mavroidis

Things to Remember

• RFC950 - old rules, over 5 years old

• Still used in some netware servers, if unsure about environment adhere to RFC950.

• RFC1812 - new rules, simplifies networking, introduces CIDR (Classless Inter-Domain routing).

• Suse 6.4 supports IPv4.

Review-RFC950

• An IP Address is broken up into three parts: the network portion, the subnet portion (optional), and the host portion. The size of the network portion is determined by the first byte of the address:

Rules - RFC950

• All hosts on the same subnet must agree on the subnet mask. Otherwise, packets actually intended for another subnet may never leave the existing subnet: a host won't give to the router a packet it thinks is destined for the local segment.

Rules - RFC950

• No two different subnets can include the same host address.

• The top and bottom host numbers are reserved; the bottom one is shorthand for the whole subnet, and the top one is the broadcast address.

• The bits in the subnet portion cannot be all ones

Valid Subnet

• Network is unique on either side of the Router. 192.168.1 + 192.168.2

Invalid Network

• Same Network resides on both sides of Router. 192.168.1

192.168.1.1 or 11000000.10101000.00000001.00000001

• Lets break this network down into 4 subnets

• That means we need to borrow 2 bits from the host octet (byte).

• Our default subnet mask is 255.255.255.0 or 11111111.11111111.11111111.00000000

• Borrowing 2 bytes gives us 128 + 64 = 192 or 11111111.11111111.11111111.11000000 or referred to as /26 or in dotted decimal notation

List of networks for 192.168.1.1

• The four networks we will get are

Network Address Calculation

• Binary and to get the network address 192.168.1.0

Host Address

• Invert the subnet and do a binary and again to get the host address 0.0.0.1

Broadcast address

• XOR to get the Broadcast address 192.168.1.63