As a network designer, you will want to allocate IPv4 address space to allow for route summarization. Large networks can grow quickly from 500 routes to 1000 and higher. Route summarization reduces the size of the routing table and, as a result, reduces the amount of route update traffic on the network. Route summarization allows the network address space to scale as the company grows.
As an example, suppose a company has assigned the following subnets to a site:
- 10.10.130.0/24 to 10.10.140.0/24 for data networks
- 10.10.146.0/24 to 10.10.156.0/24 for VoIP networks
- 10.10.160.0/24 to 10.10.166/24 for wireless networks
- 10.10.170.0/29 for access control systems
- 10.10.176.0/28 for server farm and other systems
Instead of announcing each and every subnet of this network to the WAN, the recommendation is to summarize the site with a 10.10.128.0/18 route. This subnet encompasses networks from 10.10.128.0/24 to 10.10.191.0/24, so this address block would be assigned to this site.
Planning for a Hierarchical IP Address Network
When you are IPv4 addressing for a companywide network, recommended practice dictates that you allocate contiguous address blocks to regions of the network. Hierarchical IPv4 addressing enables summarization, which makes the network easier to manage and troubleshoot.
As an example, consider the IPv4 deployment shown in Figure 1-5. Network subnets cannot be aggregated because /24 subnets from many different networks are deployed in different areas of the network. For example, subnets under 10.10.0.0/16 are deployed in Asia (10.10.4.0/24), the Americas (10.10.6.0/24), and Europe (10.10.8.0/24). The same occurs with networks 10.70.0.0/16 and 10.128.0.0/16. This lack of summarization in the network increases the size of the routing table, making the table less efficient. This lack of summarization also makes troubleshooting harder for network engineers because it is not obvious in which part of the world a particular subnet is located.
Figure 1-5 Network That Is Not Summarized
By contrast, Figure 1-6 shows a network that allocates a high-level block to each region:
- 10.0.0.0/18 for Asia Pacific networks
- 10.64.0.0/18 for Americas networks
- 10.128.0.0/18 for Europe/Middle East networks
Figure 1-6 Summarized Network
This solution provides for summarization of regional networks at area borders and improves control over the growth of the routing table.
