Is there any problem in super-dimensioning my subnetworks?

Unanswered Question
Oct 31st, 2007

I'm planning the new structure of network in my job. I thought in use some class B networks subneted, but other people told 'hey let's use a class A network So, we'll never ever have trouble with missing adresses'

Is there any problem if I have my subnetworks with much more ips than I actually need?

What is the advantage and disadvantage in use one class A network or three class b networks.

Thanks

I have this problem too.
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Hi There

Firstly I am assuming that you are talking about using private (rfc 1918)IP addresses on your network and implementing NAT towards the Internet?

If this is the case, I don't really see an issue one way or the other whether you use a class A or class B network.

Class B private address space (172.16.0.0/12) provides for over 1 million IP addresses, which I would think is more then enough for any company LAN. If you are using NAT, then this whole range is available to you to subnet as you wish.

One consideration would be your current IP addressing scheme. Are you planing to migrate all the current hosts onto these new IP address ranges or are you looking to simply add more IP addresses to your current network?

Are you currently using private address space or publicly routable addresses?

On your question on creating subnets with much more IP addresses than you actually need, the problem I see is this.

Having over large subnets can lead to too many hosts being attached to the subnet, resulting in a large amount of broadcast traffic, which will have an adverse effect on the subnet.

In my opinion it would be more prudent to put some thought into what subnets you will need immediately and how large they need to be now and then in 5 years time and dimension accordingly.

Personally the largest subnet I have seen in a live network had about 500 concurrent users but was dimensioned as a /22 subnet.

Best Regards,

Michael

ronbuchalski Wed, 10/31/2007 - 11:57

Since you said that you wanted to use some class B networks subnetted, I assume that you have multiple LAN and WAN networks that you need to address with IP.

When you refer to using Class B networks versus Class A networks, are you talking about using private IP addresses (10.0.0.0/8 or 172.16.0.0/16 - 172.31.0.0/16)? In either case, if you think about your network addressing issue in terms of classless networks, then you'll see that it really does not make any difference whether you use one or the other.

In terms of assigning subnetworks, if your network is sufficiently large that you will require many subnets (many being more than, say, fifty), it is really important to assign your subnets in such a way that you maximize the ability to summarize address space. This will reduce the size of your routing tables since you can summarize a group of subnets and only advertise the summary to the rest of the network.

For example, assume that:

1) There are ten 'regions' in your network (where 'regions' can be cities, states, countries, etc)

2) There are ten 'locations' in each 'region' (where 'locations' can be buildings, cities, districts, states, provinces, etc)

3) There are ten 'local networks' in each location

Using the above assumptions, you would have the need to assign 1000 subnetworks to your 'local networks' of your enterprise. In addition, you will need to assign subnetworks to your WAN links which interconnect these locations and regions.

While you could use either a single subnetted 10.0.0.0/8 network, or three subnetted 172.(16-31).0.0/16 networks to accomplish what you need, I would recommend assigning your addresses from a single 10.0.0.0 network because it offers you the ability to summarize addresses more intuitively than combining multiple /16 networks into supernets.

All summarizing within the 10.0.0.0 address space will still be a prefix length longer than the base network dimension of /8. Combining three /16s will mean supernetting with /15, and will not give you very much granularity when it comes to summarizing your address space.

Hope this helps.

Ron

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