Inhoud
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 | 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
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