Introduction to network summarization terminology

The term “route aggregation” is just another way of saying “network summarization.” Supernetting, on the other hand, is the process of taking multiple networks and making a single larger network (very similar to summarization and aggregation). But what, specifically, are summarization and aggregation?

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Network summarization is the act of taking two or more IP networks and using a single IP network to represent them all. This is possible because of classless inter-domain routing (CIDR). CIDR eliminates the need for a fixed-length subnet mask (FLSM) associated with our IP networks. This goes back to the old standard of having to have a certain subnet mask for classes of IP networks. Because everyone uses CIDR today, we can use variable-length subnet masks (VLSMs). This means we have the luxury of adding or subtracting bits from our subnet mask to allow us to either subnet our network further (by adding bits) or supernet our network (by taking bits away). Let’s find out more.

What is network summarization (aka route aggregation)?

Say that I have IP networks 1.1.1.0 /24, 1.1.2.0 /24 and 1.1.3.0 /24. All of these networks have the subnet mask of 255.255.255.0 (hence, the /24). (See Binary-to-decimal conversion for more information on how this is calculated.) This means that if I take away bits from the subnet mask, I can encompass both of these networks with a single IP network and subnet mask. For example, I could say that these networks are represented by the IP address 1.1.0.0 with the subnet mask 255.255.0.0. With that statement, I have summarized or aggregated the three networks and am now representing them with a single IP address and subnet.

However, this is just an example; I did this very inefficiently. I actually summarized many other networks besides the three in question. I summarized all networks that fit into this range 1.1.{0-255}.{0-255}. To summarize only my three networks as efficiently as possible, I would use IP network 1.1.0.0 with subnet mask 255.255.252.0. This encompasses all three IP networks: 1.1.1.0, 1.1.2.0 and 1.1.3.0.

This can, of course, be done on a network calculator, but it is not hard to do by hand either. Here is an example:

Third octet of each IP and subnet mask

000000 01 = 1
000000 10 = 2
000000 11 = 3
111111 00 = 252

Using network summarization on a router

No matter the name that you give it, you need to know how to use network summarization/aggregation on a router. To do this, you will represent the IP networks with a single IP address and either a subnet mask or a wildcard mask.

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Here is an example of using a subnet mask to summarize the same three networks in OSPF:

Router(config)# router ospf 1
Router(config-router)# area 1 range 1.1.0.0 255.255.252.0

As you can see, I just made an entry to summarize all three networks using the “range” command. The range command is used to summarize other OSPF networks. The same example can be applied to summarizing networks received from a different router protocol like Routing Information Protocol (RIP). Here’s an example with the “summary-address” command:

Router(config-router)# summary-address 1.1.0.0 255.255.252.0

In both these cases, we have summarized or aggregated the network.

What are wildcard masks?

Wildcard masks are just another way to represent a subnet mask. Don’t let how they look scare you off. You can easily convert a subnet mask into a wildcard mask. A wildcard mask is simply all the 1s in a subnet mask (when in binary form) turned into 0s, and all the 0s turned into 1s.

Here’s an example:

Say that I have the same subnet mask as above, 255.255.252.0. This subnet mask in binary is:

11111111 11111111 11111100 00000000

To convert this to a wildcard mask, I simply turn all 1s into 0s and all 0s into 1s, then convert back to decimal.

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Here’s the conversion to a wildcard in binary:

00000000 00000000 00000011 11111111

Then, here’s the conversion of that same wildcard in binary to decimal:

0.0.3.255

Wildcard masks are used in access lists. So, say that I wanted to permit the same three networks we listed above and deny all others. Here is an example of an access control list (ACL) that would do that, using a wildcard mask:

Router(config)# access-list 1 permit 1.1.0.0 ?
  A.B.C.D  Wildcard bits
  log      Log matches against this entry
  

Router(config)# access-list 1 permit 1.1.0.0 0.0.3.255
Router(config)# access-list 1 deny any

With this ACL, all three of our networks (1.1.1.0, 1.1.2.0 and 1.1.3.0) are permitted and all other networks are denied. For video training on subnetting and supernetting, checkout my favorite Cisco CCNA / CCDA video training!

Article summary

Here is what we have learned:

• An introduction to network summarization terminology, including network summarization, route aggregation, supernetting, CIDR and VLSM.
• How to use network summarization on a router with the OSPF routing protocol (in two different ways).
• What wildcard masks are, and how to use them in a Cisco IOS access list.

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Allocating IP addresses

When you define which IP addresses will be on which network, you are not only setting a precedent that will be difficult to change, you are limiting the size of your network. This is because IP networks/subnets have limited sizes. For example, a Class C network (like 192.168.1.0 /24) can have up to 254 usable computers. That may be enough for your network today, but it may not be enough for your network next year. Of course, the ideal time to properly size these IP networks is when you design the network. Your design is only as good as the information you have at hand. Let’s say that you expect each network to have 125 computers and not grow beyond 254 computers. When you configure your routers and design an IP address scheme, you will assign a Class C IP address network to this network. If after six months the device count needs to go up to 400, however, you will have to make a change in your design. You will have a couple of choices.

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Sizing your network

To properly size your network from the beginning, use the host’s formula. This formula says that for the number of zeros in the subnet mask when converted to binary, take 2 to the power of that number, minus 2, and you will see the number of possible hosts when using that subnet mask. This can be done on a subnetting calculator, of course, or with the help of handy subnetting charts. (See IP addressing and subnetting: Calculate a subnet mask using the host’s formula for more information.)

Optimize network space for IP addresses

If possible, it is important to know where your company is going with this location — meaning, how many networked devices will be at this site? Don’t forget to include laser printers, servers and other networked managed devices (UPS systems, for instance).Once you know that, you need to try to find out what the expected growth is for this site. Will the number of devices eventually double? Often, this can be limited by the physical size of the office. If all you have is a small lot with a single building, and every office is already filled with a PC, there isn’t physical space to add many more devices.
In having all this information, you will know how many IP addresses you may need in the future, so you can select the proper IP address space.
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NAT and the Internet

With private networking (RFC1918), you have many IP addresses available for your internal use. To access the Internet from those IP addresses, you just have to perform network address translation (NAT) Let’s look at an example. Say your company has 70 locations with 50 devices each. One option would be to allocate 126 usable IP addresses with a /25 subnet mask (or 255.255.255.128), still allowing enough IPs for the network to double in size and using the IP address space as efficiently as possible. This would allow you enough growth to have some 130,000 networks of this size. I doubt that your company would grow beyond that number of sites.

On the other hand, you could allocate more IP addresses per site and have fewer networks. Say you allocated 65,000 hosts to this subnet with a /16 mask (or 255.255.0.0). That would leave you the ability to grow to 256 networks of this size, but this seems like a huge waste of IP addresses.

Finally, you could allocate many more IP addresses than you might need, break the network at the octet boundary, and still have the ability to add many networks. Let’s say you allocated a /24 subnet mask (or 255.255.255.0). That would give you 254 usable hosts per network (you could grow 5x) and the ability to have 65,536 subnets (or locations). This seems like a more reasonable approach.

In my opinion, the worst thing you could do is to allocate a /26 subnet mask (or 255.255.255.192) and give yourself only 62 usable hosts on each network. The 50 hosts you have would almost certainly outgrow this at some site.

Options when you are out of IP addresses

Let’s say that someone allocated too few IP addresses to a network. What are your options?

Option A: Re-address the network
The first option to resolve a network that is out of IP addresses is to allocate a larger IP subnet (more addresses) and change the subnet mask on all devices. Although this costs nothing monetarily, it could cost quite a bit in time. You don’t want to let your LAN get too big, however. The more devices, the more broadcasts you will have. Eventually, over perhaps 300+ devices, your network will begin to have performance problems from the devices’ having to process so many broadcasts that weren’t meant for them.

Option B: Add a second network or VLAN
Another option would be to add a second network. Say you have one network of 192.168.1.0/26 with 62 usable hosts. Suppose you have 60 hosts and need more. You could add network 192.168.1.64 and use hosts 192.168.1.65-192.168.1.126. To do this and allow the two networks to communicate, you would add a second LAN interface on your router and route between these two networks. Each network would have a different IP gateway, and you might have different DHCP servers on each network.

The problem with doing this is that the devices on each network must be physically cabled separately, come back to a single switch, and then be connected to their respective interfaces on the router. In a larger office, this can be very difficult.

A great solution to this as the number of devices grows is to use VLANs. With VLANs, the devices in the different networks could be anywhere, still be on their network, and still communicate to the devices on the other VLANs.

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