7.VLSM-Gen-Routes

	/23	/24	/25	/26	/27	/28	/29	/30
Number of Addresses	512	256	128	64	32	16	8	4

General Routing Table for Router A

I’ll be basing this of VLSM Example 1 under topic 7 in your E-learn

Our subnets arranged by Network Address:

Network	Network Address	Mask	Range(network to broadcast) of Forth octet(x.x.x._) in binary
C	192.168.160.0	/25	0000 0000 to 0111 1111
A	192.168.160.128	/26	1000 0000 to 1011 1111
B	192.168.160.192	/27	1100 0000 to 1101 1111
AB	192.168.160.224	/30	1110 0000 to 1110 0011
AC	192.168.160.228	/30	1110 0100 to 1110 0111
BC	192.168.160.232	/30	1110 1000 to 1110 1011

Red: Network portion of the binary (Masked bits)

The smallest mask of the 3 is ‘/27’ and it belongs to network B
Network B is 192.168.160.192/27
Network B’s address range(network to broadcast): 192.168.160.192-223
We can see that networks AB and BC which start from x.x.x.224 and x.x.x.232 respectively, do not fall into Network B’s range (obviously)
So let’s take Network B’s address and shorten the mask from /27 to /26
Doing so, we get the new range of 192.168.160.192-255
1. We went from (/27): 1100 0000 to 1101 1111
2. To (/26): 1100 0000 to 1111 1111
Now this new range matches all the possible addresses of networks B, AB and BC!

Something feels off… but lets try and make the routing table:

192.168.160.0-255 (/24) & 192.168.160.192-255 (/26)
They overlap.
This is not always a problem… But in this case it is. (see bellow for an example of when overlaps are okay)
Remember that the router will choose the most specific route!
AC’s network address is: 192.168.160.228
Instead of following the route we want (192.168.160.0-255 (/24))
It will follow the more specific route (192.168.160.192-255 (/26))
and we don’t want that… So! we have to choose to get rid of one of them…
So to maximise efficiency, we get rid of the general route that matches the least networks (C, AC)!
Here’s a table that visualises this better

Thus, giving us:

Networks	Target Addr	Mask	Port
AC	192.168.160.228	/30	pa2
A	192.168.160.128	/26	pa0
B, AB, BC	192.168.160.192	/26	pa1
C	192.168.160.0	/25	Pa2
Internet	0.0.0.0	/0	pa3

This table is ordered by most specific first for the sake of clarity. But, the order doesn’t actually matter, write it in any order that u wish.

Networks: The networks routed by this Target Address

Target Addr: The Address used to match an IP and specify its route

Port: The port the route should take

Here’s what the full routing table looks like for our example router D:

Network	Network Addr	Addr Range	Port
A	192.168.0.0/26	192.168.0.0-63	pd0
B	192.168.0.64/27	192.168.0.64-95	pd1
C	192.168.0.96/27	192.168.0.96-127	pd1
D	192.168.0.128/27	192.168.0.128-159	pd0
E	192.168.0.160/27	192.168.0.160-191	pd0

This table is ordered by Increasing Network Address, I recommend ordering the full routing tables this way, as it makes it easier to generalise.

Let’s do the generalisation for Networks attached to pd0 (A, D, E):

Find a general address that can encapsulate A, D and E
- We can’t use 192.168.0.0/25 because that’s only ranges: 192.168.0.0-127
- So we have to use 192.168.0.0/24. Which ranges: 192.168.0.0-255
Find a general address that can encapsulate B & C
- 192.168.0.64/26 ranges: 192.168.0.64-127
  - 192.168.0.64 → 192.168.0. 0100 0000
  - 192.168.0.127 → 192.168.0. 0111 1111

So! let’s make the Generalised Routing Table:

Network	Addr	Mask	Matches Addresses in Range	Port
B, C	192.168.0.64	/26	192.168.0.64-127	pd1
A, D, E	192.168.0.0	/24	192.168.0.0-255	pd0

This is perfectly fine! Our general address for network B & C only matches networks B & C.