EE5412 IP_Subnet0.doc

EE5412 [IP_Subnet0.doc]

Subnetting
Assume
1. class B network of 128.6.0.0 has been assigned
2. two LANs or networks located in two different physical locations
3. main router has one external and two local interfaces, each local interface connects to a local network



‘a’ assign IP in random to host, routing table ‘b’ has to be updated.

‘b’ create two subnets with starting address 128.6.1 and 128.6.5

Motivation for Subnetting
Grouping address assignment by using geographic locations makes the routing table much simpler.

Effect of Simplified routing table –

Routers share routing table with others to let each other know what is connected.
Larger table places more demand [ie.Traffic] on the network as it is periodically transmitted.
Looking up entries is also time-consuming with larger table.

Subnet creates traffic isolation and security between work groups.










Example
Assume 5 bits host in the following IP

IP address 1101, 0110, 1001, 0010, 1110, 0111, 0100, 1101 214.146.231.77
Subnet Mask 1111, 1111, 1111, 1111, 1111, 1111, 1110, 0000 255.255.255.224
Subnet address 1101, 0110, 1001, 0010, 1110, 0111, 0100, 0000 214.146.231.64
Host number 0, 1101 13
Broadcast address 1101, 0110, 1001, 0010, 1110, 0111, 0101, 1111 214.146.231.95


IP address and its mask 214.146.231.77/27 27 is the mask in subnet

Loss of IP address
Each subnet occupy 1 IP address space for denoting subnet address [ie. All 0] and
1 IP address space for subnet broadcasting address [ie. All 1]

In the above example : IP address 214.146.231.64 and
214.146.231.95 is not available for host address allocation.
=> more subnet => more unavailable IP address for host





Example
An organization with class C network, 200.200.200.0 network assigned.
At home office , estimated max of 50 interface

Three branch office each with 20 interface
Lease line connects at each branch to the router at home office. Each line need IP address with its own subnet.


With class C, the last 8 bit in IP address is available for subnet. We can have 6 subnet and each subnet has 30 interface

000x,xxxx cannot use [200.200.200.0]
001x,xxxx branch 1 1 subnet address, 1 broadcast address, 2 link address , 20 host address
010x,xxxx branch 2
011x,xxxx branch 3
100x,xxxx home office
101x,xxxx home office
110x,xxxx not used
111x,xxxx cannot use [200.200.200.255]

Subnet mask 255.255.255.0

Routing table 200.200.200.32 branch 1 link
200.200.200.64 Branch 2 link
200.200.200.96 branch 3 link
200.200.200.128 home office link
200.200.200.160 home office link
200.200.200. 192 default link