This article is the last in a series of IP subnetting articles. While the other articles were focused on reviewing the concepts behind binary math and subnet ranges, this article takes a look at the practical implementation of these examples ranges on Cisco IOS as well as the configuration of a basic Routing Information Protocol (RIP) network showing a complete routing table between all organizational departments.
As discussed in the previous article, the Acme University is planning to deploy an IP network in their main campus location and is wants to ensure that it is developed in a way to ensure that easy growth is possible. As the university operates strictly online, its main campus location includes only a single physical building with 4 floors and 8 total departments: Administration, Admissions, Financial Aid, Business school, Liberal Arts school, Internet Technology school, Science school, and History school. Each of the different departments needs to be separate and have their own IP address space. The host address space that has been allocated to the University by their Internet Service Provider (ISP) is 172.16.0.0/23. Each of the different departments requires at least 40 different usable addresses and at least 10 extra addresses allocated for future growth. The University uses the 192.168.1.0/24 network to connect to the ISP and is allowed to control the assignment of these addresses with the exception of 192.168.1.1 which has been reserved by the ISP’s router.
Figure 1 below shows a graphic representation of the suggested equipment layout within the organization’s main campus building.
Figure 1 Acme University Main Campus Building
Calculated IP Addressing Ranges
The previous article reviewed the process that is required to come up with an appropriate IP Addressing range based on a set of requirements. Each of the ranges required at least 50 addresses; based on this, the 255.255.255.192 (/26) subnet mask selected allocating 3 additional subnetwork bits to create 8 different networks for the departments and allocating 6 host bits for a total of 64 host addresses per department. The following list shows the IP address host addresses allocations that were developed:
- Administration: 172.16.0.01718.104.22.168
- Admissions: 172.16.0.64172.16.0.127
- Financial Aid: 172.16.0.128172.16.0.191
- Business School: 172.16.0.192172.16.0.255
- Liberal Arts School: 172.16.1.01722.214.171.124
- Internet Technology School: 172.16.1.64172.16.1.127
- Science School: 172.16.1.128172.16.1.191
- History: 172.16.1.192172.16.1.255
Each of the routers on each of the floors is to be assigned the lowest usable address in each of the host ranges.
To connect to each other and provide connectivity to the ISP, each of the routers will be assigned an IP address from the 192.168.1.0/24 range; the address will be equal to 192.168.1.X0, where X is the floor of the router.
IP Address Configuration
The first router to be configured is Floor1; the Floor1 router connects the Administration and Admissions departments as well as ties together all of the other routers together. According to Figure 1, Floor1 has three different interfaces that need to be configured; this configuration is shown in Figure 2:
Figure 2 Floor1 Configuration
The router on the second floor connects to the Financial Aid department and to the Business school. According to Figure 1, like Floor1, Floor2 also has three different interfaces that need to be configured; this configuration is shown in Figure 3:
Figure 3 Floor2 Configuration
The router on the third floor connects to the Liberal Arts and Internet Technologies schools. According to Figure 1, like Floor1 and Floor2, Floor3 has three different interfaces that need to be configured; this configuration is shown in Figure 4:
Figure 4 Floor3 Configuration
The router on the fourth floor connects to the Science and History schools. As we saw in Figure 1, like Floor1, Floor2, and Floor3, Floor4 has three different interfaces that need to be configured; this configuration is shown in Figure 5:
Figure 5 Floor4 Configuration
Without any additional configuration, each of the routers would be able to communicate with each other and the direct departments or schools that they are connected to; however, they would be unable to communicate with any of the other departments or schools that are not directly connected to. As Figure 6 shows, the routing table for Floor1 only contains the information for the 172.16.0.0/26, 172.16.0.64/26 and 192.168.1.0/24 networks.
Figure 6 Floor1 Routing Table
To enable each of the floor routers to speak with each other, either several static routes need to be configured or a dynamic routing protocol needs to be configured. For the purposes of this article, a basic RIP routing process will be configured on each device. By default, when RIP is configured on a device, version 1 is used. The problem with this is that RIP version 1 does not support classless networks as used in this article, version 2 does support classless networks and because of this, it will be used. The other problem with RIP is that by default it automatically summarizes networks; to get around this for this article it will be disabled.
Figure 7 below shows the RIP configuration of the Floor1 router:
Figure 7 Floor1 RIP Configuration
Figure 8 below shows the RIP configuration of the Floor2 router:
Figure 8 Floor2 RIP Configuration
Figure 9 shows the RIP configuration of the Floor3 router:
Figure 9 Floor3 RIP Configuration
Figure 10 shows the RIP configuration of the Floor4 router:
Figure 10 Floor4 RIP Configuration
Once RIP has been allowed to converge, the routing table for Floor1 has changed considerably; Figure 11 below shows the complete routing table for Floor1:
Figure 11 Floor1 Complete Routing Table
With this configuration, all of the routers and host devices are able to connect to each other; RIP maintains the most efficient routes that will be inserted into the routing table.
This article finishes up the series of articles about subnetting; hopefully the content of each of these articles has enabled the reader the ability to understand the concepts required to subnet IPv4 networks. This article was intended to take the theory that was covered from the previous three articles and put a practical face on it from the perspective of Cisco routing equipment. Keep in mind that the subnetting knowledge that was covered here is not specific to Cisco; each of these IP ranges that was calculated will work the same regardless of the manufacturer of equipment. Make sure to allot the time to practice subnetting and enable a good understanding of the basic math behind it, this core knowledge will enable an easy transition into developing IPv4 and IPv6 subnets going forward.