IP Subnetting Case

Case

Imagine an organization geographically divided into three locations:

Location A: 120 ip-addresses needed
Location B: 40 ip-addresses needed
Location C: 32 ip-addresses needed

All systems in your organization need direct Internet access.
(no proxy, no NAT (Network Address Translation))

What network class do you need from your ISP?
(ISP: Internet Service Provider)
Configure your network given the ip-range provided to you by the ISP.

Solution

1. What network class do we need from the ISP?

1.1 Determine the number of hostbits needed in each network.

Location	Formula	Hostbits	Max. Hosts
A	2^h -2 >= 120	7	126
B	2^h -2 >= 40	6	62
C	2^h -2 >= 32	6	62

1.2 Aggregate the subnets

Aggregated subnets show up as one larger network to the outside world!

n: Network bit
h: Host bit

Location	Hostbits	Network
B	6	A . B . C . nnhh_hhhh
C	6	A . B . C . nnhh_hhhh
B+C	7	A . B . C . nhhh_hhhh

Location	Hostbits	Network
A	7	A . B . C . nhhh_hhhh
B+C	7	A . B . C . nhhh_hhhh
A+B+C	8	A . B . C . hhhh_hhhh

Conclusion: We need a C-class ip-network: A . B . C . 0 /24

Note:
According to the rules subnets of all zeros or all ones are not allowed.
So, when splitting a network into two subnets you actually have to split the network using two subnet bits, creating subnets: “01”, “10”.
The big drawback is the waste of ip address space. Since subnets “00”and “11” are illegal, you have to throw away half of the address space! Many organizations can't last such a waste.

2. Configure the Network

The ISP gracefully provided us: 195 . 72 . 17 . 0 /24

Location A
Decimal	Last Octet	Item
255 . 255 . 255 . 128	1000_0000	Netmask
195 . 72 . 17 . 0	0000_0000	Network
195 . 72 . 17 . 1	0000_0001	First Host
195 . 72 . 17 . 126	0111_1110	Last Host
195 . 72 . 17 . 127	0111_1111	Broadcast

Location B
Decimal	Last Octet	Item
255 . 255 . 255 . 192	1100_0000	Netmask
195 . 72 . 17 . 128	1000_0000	Network
195 . 72 . 17 . 129	1000_0001	First Host
195 . 72 . 17 . 190	1011_1110	Last Host
195 . 72 . 17 . 191	1011_1111	Broadcast

Location C
Decimal	Last Octet	Item
255 . 255 . 255 . 192	1100_0000	Netmask
195 . 72 . 17 . 192	1100_0000	Network
195 . 72 . 17 . 193	1100_0001	First Host
195 . 72 . 17 . 254	1111_1110	Last Host
195 . 72 . 17 . 255	1111_1111	Broadcast