Strategies for Successful Addressing
You can allocate addresses in four ways:
First come, first serve— Start with a large pool of addresses and hand them out as they are needed.
Politically— Divide the available address space up so every organization within the organization has a set of addresses it can draw from.
Geographically— Divide the available address space up so that each of the organization's locations has an office that has a set of addresses it will draw from.
Topologically— This is based on the point of attachment to the network. (This may be geographically the same on some networks.)
First Come, First Serve Address Allocation
Suppose you are building a small packet switching network (one of the first) in the 1970s. You don't think this network will grow too much because it's restricted to only a few academic and government organizations, and it's experimental. (This prototype will be replaced by the real thing when you're done with your testing.)
No one really has any experience in building networks like this, so you assign IP addresses on a first come, first serve basis. You give each organization a block of addresses, which seems to cover their addressing needs. Thus, the first group to approach the network administrators for a block of addresses receives 10.0.0.0/8, the second receives 11.0.0.0/8, and so on.
This form of address allocation is a time-honored tradition in network design; first come, first serve is, in fact, the most common address assignment scheme used. The downside to this address allocation scheme becomes apparent only as the network becomes larger. Over time, a huge multinational network could grow to look like the Internet—a mess in terms of addressing. Next, look at why this isn't a very good address allocation scheme.
In Figure 2-10, the network administrators have assigned addresses as the departments have asked for them.
Figure 2-10. First Come, First Serve Address Allocation
This small cross-section of their routers shows:
Router A has two networks connected: 10.1.15.0/24 and 10.2.1.0/24
Router B has two networks connected: 10.2.12.0/24 and 10.1.1.0/24
Router C has two networks connected: 10.1.2.0/24 and 10.1.41.0/24
Router D has two networks connected: 10.1.40.0/24 and 10.1.3.0/24
There isn't any easy way to summarize any of these network pairs into a single destination, and the more you see of the network, the harder it becomes. If a network addressed this way grows large enough, it will eventually have stability problems. At this point, at least eight routes will be advertised into the core.
Addressing by the Organizational Chart (Politically)
Now, start over with this network. Instead of assigning addresses as the various departments asked for them, the network administrators decided to put some structure into their addressing scheme; each department will have a pool of addresses to pull networks from:
Headquarters: 10.1.0.0/16
Research: 10.2.0.0/16
Quality: 10.3.0.0/16
Sales: 10.4.0.0/16
Manufacturing: 10.5.0.0/16
With this addressing scheme in place, the network now looks like Figure 2-11.
Figure 2-11. Addressing on the Organizational Chart
Now, there may be some opportunities for summarization. If 10.1.3.0/24 isn't assigned, it might be possible to summarize the two headquarters networks into one advertisement. It's not a big gain, but enough little gains like this can make a big difference in the stability of a network.
In general, though, this addressing scheme leaves you in the same situation as the first come, first serve addressing scheme—the network won't scale well. In Figure 2-11, there will still be at least seven or eight routes advertised into the core of the network.
Addressing Geographically
Once again, you can renumber this network; this time assign addresses based on the geographic location. The resulting network would look like Figure 2-12.
Note the address space has been divided geographically; Japan is assigned 10.2.0.0/16, the United States is assigned 10.4.0.0/16, and so on. While it's probable that some gains can be made using geographic distribution of addresses, there will still be a lot of routes that cannot be summarized.
Figure 2-12. Addressing by Geographic Location
Just working with the networks illustrated here, you can summarize the two US networks, 10.4.1.0/24 and 10.4.2.0/24 into 10.4.0.0/16, so Router A can advertise a single route into the core. Likewise, you can summarize the two Japan routes, 10.2.1.0/24 and 10.2.2.0/24, into 10.2.0.0/16, and Router D can advertise a single route into the core.
London, however, presents a problem. London Research, 10.1.2.0/24, is attached to Router C, and the remainder of the London offices are attached to Router B. It isn't possible to summarize the 10.1.x.x addresses into the core because of this split.
Addressing by Topology
The most effective way of making certain that routes can be summarized is to assign addresses based on the router to which the network is attached or, rather, the topology of the network. Addressing this network based on the topology results in Figure 2-13.
Figure 2-13. Topological Address Assignment
Summarization can now be configured easily on Router A, Router B, Router C, and Router D, reducing the number of routes advertised into the rest of the network to the minimum possible. This is easy to maintain in the long term because the configurations on the routers are simple and straightforward.
Topological addressing is the best assignment method for ensuring network stability.
Combining Addressing Schemes
One complaint about assigning addresses topologically is it's much more difficult to determine any context without some type of chart or database—for example, the department to which a particular network belongs. Combining topological addressing with some other addressing scheme, such as organizational addressing, can minimize this.
Because an IP address is made up of four octets, it's possible to use the left two octets for geographic numbering and the third for departments (or some other combination). For example, if you assign the following numbers to the following departments:
Administration: 0-31
Research: 32-63
Sales: 64-95
Manufacturing: 96-127
and the following attachment points to the following numbers:
Router A: 4
Router B: 1
Router C: 3
Router D: 2
some sample addresses would be:
Administration off of Router A: 10.4.0.0/24 through 10.4.31.0/24
Research off of Router A: 10.4.32.0/24 through 10.4.63.0/24
Manufacturing off of Router C: 10.3.96.0/24 through 10.3.127.0/24
Combining addressing schemes will allow less summarization than assigning addresses strictly based on the connection point into the network, but it may be useful in some situations.