|
Home > Archive > CCNP > August 2005 > BSCI summerization question
You are viewing an archived Text-only version of the thread.
To view this thread in it's original format and/or if you want to reply to
this thread please [click here]
| Author |
BSCI summerization question
|
|
| zxmjkxz 2005-07-26, 8:50 am |
| subnets 172.16.12.0/24, 172.16.13.0/24, 172.16.14.0/24, and 172.16.15.0/24 are summerized into 172.16.12.0/22..
172.16.12.0/22
11111111.11111111.11111100.00000000
128,64,34,16,8,4,2,1
Question:
I'm not sure why it goes to 22 from 24. Is it because 2^2=4, and since the range is 4
subnets (12-15) then thats why 2 bits are taken?? thank you... | |
| NetNut 2005-08-03, 4:14 am |
| When thinking about summarization, you need to think in terms like: "what 'bigger' subnet can be used to include all my subnets which are contiguous (or nearly contiguous)?" In other words, even though all your actual subnets are /24 in size, you want to summarize these routes to relieve traffic and simplify routing tables.
Specifically, in your example you need to look at the third octets! You can see that the ".12" ".13" ".14" ".15" and ".16" subnets could all be included in one 'giant' subnet of /22.
For binary example:
172.16.12.0 is
10101100.00010000.00001100.00000000
172.16.13.0 is
10101100.00010000.00001101.00000000
172.16.14.0 is
10101100.00010000.00001110.00000000
172.16.15.0 is
10101100.00010000.00001111.00000000
Note that the last TWO binary digits in the THIRD octet are 00, 01, 10, and 11 respectively? This might click to you as a four network subnet? If you look at the four networks as one, then you can use the /22 mask.
Now this does NOT magically make them all one actual network, however, you’re your router is advertising to other routers, it can say "Hey! If you're looking to access any network that is 172.16.12.0/22 (all four networks in your example), then talk with me!" This keeps it from advertising all four routes individually. Hopefully this makes sense to you – review the subnet listing in binary until you see what I'm talking about.
Good Luck!
NetNut | |
| PhillyD 2005-08-12, 8:38 am |
| I find that the easy way to work it out is that the first 22 bits of the address are the same whichever of the 4 subnets you look at, and if the address was made up of just the first 22 bits it would be 172.16.12.0 and obviously the mask is /22 -which makes up the summarised address. | |
| zxmjkxz 2005-08-31, 4:00 am |
| thank you guys, I appreciate it |
|
|
|
|