subnetting question

Unanswered Question
Jul 3rd, 2009
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Hi all

when subnetting and summary addressing, if I wanted to summarize 4 networks, would I use /30, bit If I wanted 4 hosts, I wouuld need to use /29 ?

when summarizing, im a little unsure if I need to stop before i get to the number I need, for example, i mark down

128 64 32 16 8 4 2 1 , for 4 hosts would it be /29, but for summarizing it would be /30 ?

please help

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jfisher000 Fri, 07/03/2009 - 08:07
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I will try to assist you. I do not really understand what you are asking with "summarize 4 networks." However, if you wanted at least 4 hosts you could use /29.

For example, let us look at

network address =

subnet mask = (/29)

Subnets? 248 in binary = 1111 1000. 2^5=32 possible subnets

hosts? 2^3-2 = 6 HOSTS you get that by taking the zeros at the end of the binary. 1111 1000. Keep in mind that you subtract 2 due to the broadcast and subnet ID (network ID)

Valid subnets? 256-248 = 0,8,16,24

So the valid hosts for network 0 would be 1-6 and the subnet ID would be 0 and the broadcast would be 7.

xcz504d1114 Fri, 07/03/2009 - 08:12
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Summarization is taking several networks and presenting it as 1 network.

For instance, if I had 2 networks, both networks had 250 users, so I would use a /24 ( subnet mask for each network.

The networks would look like this:

If I had a routing protocol advertsing these networks a neighboring router would see both routes in his routing table.

To make efficient use of memory and CPU cycles we could summarize those.

We would summarize those 2 networks like this:

The /23 would encompass both networks, the and the

If I added a third network later, and i still wanted to summarize:

I would add

And change my summary to

The problem with this summary address is that it covers more than just the networks that I own.

So if there is another router that is using the in my routing domain, I could potentially be recieving his data. As long as he is advertising a more specific route than my summary route (more specific means a longer subnet mask IE: /24 is longer than /22) then everything is ok. But if his network goes down, any traffic destined to him now gets forwarded to my router.

A summary address is a less specific route that you would use to advertise many smaller networks.

Another quick example:

Could be summarized with a

In your example, since you are just using 1 network that has 4 hosts, there is no need to summarize. However if you had multiple networks containing 4 hosts, then you could summarize if they were consecutive networks. will allow hosts - 6 (6 hosts not 4).

You could then have a second network: to allow hosts - 14

You could then summarize those 2 networks with: (network range of -



Leo Laohoo Fri, 07/03/2009 - 19:19
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Hiya Carl,

If you use /30, the IP Addresses are:

1 < - Subnet (you can't use this, of course)

2 < - Valid IP Address to use

3 < - Valid IP Address to use

4 < - Broadcast (you can't use this either)

If you need IP Addresses to use for 4 host you need the next one up, a /29. Here's the breakdown:

1 < - Subnet (Can't touch this!)

2 to 6 < - Valid IP Address to use up to 5 hosts

7 < - Broadcast (Can't touch this!)

Hope this helps.


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