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))

  1. What network class do you need from your ISP?
    (ISP: Internet Service Provider)
  2. 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 2h -2 >= 120 7 126
B 2h -2 >= 40 6 62
C 2h -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

.

Copyright© 2005 Integrated Services; Tux4u.nl
Author: ing. J.M. Waldorp; subnetting-case 2005-02-14

 
networking/tcpip/subnetting-case.txt · Laatst gewijzigd: 2018/12/31 14:11 (Externe bewerking)
 
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