CCNA Exploration: Routing Protocols and Concepts Chapter 6 Case Study Scenario: As you know, Tred has 3 physically separate offices. From a network perspective, they are identified as Branch 1 (B1), Branch 2 (B2) and Main (M). Main was the first office created and it has a connection to Internet. As shown on the topology above, B1 and B2 reach Internet via M. Tred hired 2 512Kb WAN links from the telephony company to interconnect all 3 sites. The first WAN link connects B1 to B2 and the second, B2 to M. A third 2Mb link connects M to the internet. In order to provide a second redundant path to M, a third 1Mb/s link was hired. The new link connects B1 directly to M. M is responsible for providing Internet connectivity to all Tred Inc. sites and for routing packets to/from Network 3 (192.1 The WAN links are point-to-point links but currently an entire class C network is being wasted on those links.
After a study on Tred network, you create a better IP addressing scheme which is based on Variable Length Subnet Mask. VLSM allows a better use of valid IP addresses while facilitating future route summarization implementation. You design your IP new addressing scheme to use only one class C range to address the entire Tred’s network. It must guarantee 2 valid IP addresses to each WAN link and 30 valid IP addresses on the networks which supports user PCs.
Question 1: How many networks must be addressed within Tred’s network environment? Specify how many valid IP addresses each network must have. Answer: 6 Networks: 3 serial links and 3 Ethernet user networks. Serial Links: requires 2 valid addresses. Ethernet Segments: 30 valid addresses (according to Tred’s people).
Step 1 – Designing The Address Scheme Since Tred has Ethernet segments holds up to 30 devices, you decide to work with a /27 mask.
CCNA Exploration: Routing Protocols and Concepts Chapter 6 Case Study Question 2: How many networks and how many hosts are possible when segmenting a class C network with a /27 subnet mask? Answer: /27 mask leads to 8 sub-networks with 32 addresses each. (only 30 of these 32 addresses are valid addresses) Based on the class C address 192.168.50.0/24 given to Tred, you create the following sub-networks: 192.168.50.0/27 sub-netted with /27 mask: 1st subnet: 192.168.50.0/27 – 192.168.50.31/27 2nd subnet: 192.168.50.32/27 – 192.168.50.63/27 3rd subnet: 192.168.50.64/27 – 192.168.50.95/27 4th subnet: 192.168.50.96/27 – 192.168.50.127/27 5th subnet: 192.168.50.128/27 – 192.168.50.159/27 6th subnet: 192.168.50.160/27 – 192.168.50.191/27 7th subnet: 192.168.50.192/27 – 192.168.50.223/27 8th subnet: 192.168.50.224/27 – 192.168.50.255/27
Once the subnets are created, you map the first 3 subnets to the 3 Ethernet segments shown on the topology. The mapping is as follows: 1st Subnet (192.168.50.0/27) -> Network 1 2nd Subnet (192.168.50.32/27) -> Network 2 3rd Subnet (192.168.50.64/27) -> Network 3
Once the Ethernet segments are addressed, it is time to created subnets to be applied to the serial links and you pick one of the unused remaining sub-networks to subnet it again. You choose the last subnet (192.168.50.224/27) to work with and because the serial link will only have 2 addresses (each end of the point-to-point link) to choose to use a /30 subnet mask.
Question 3: How many networks and how many hosts are possible when segmenting a /27 network with a /30 subnet mask? Answer: When applied to a /27 mask network, a /30 mask leads to 8 sub-networks with 4 addresses each (only 2 of these 4 addresses are valid addresses). By working with a /30 subnet mask, you achieve 4 addresses per network: the first address is the network address and can’t be assigned to devices. The second and third addresses are valid and can be assigned to devices. The fourth address is the broadcast address and can’t be assigned to devices. You subnet the last /27 subnet and document the result as follows:
Since each of the subnets above is an independent subnet, you map the first 3 to the 3 serial links of Tred’s network. The mapping created by you is also documented below: 1st Subnet (192.168.50.224/30) -> Network 5 2nd Subnet (192.168.50.228/30) -> Network 6 3rd Subnet (192.168.50.232/30) -> Network 7
The second and third addresses of each /30 subnet will be assigned to the router interfaces. The topology containing updated addresses is shown below:
CCNA Exploration: Routing Protocols and Concepts Chapter 6 Case Study VLSM Updated Topology
Step 2: Assign Addresses To The Routers Interfaces: Once the IP address scheme is ready, it is time to actually assign such addresses to the routers interfaces. You connect to the routers, issue the necessary commands and document them for future reference.
CCNA Exploration: Routing Protocols and Concepts Chapter 6 Case Study M(config)# int fa0/0 M(config-if)# ip address 192.168.50.65 255.255.255.224 M(config)# int se0/1 M(config-if)# ip address 192.168.50.229 255.255.255.252 M(config)# int se1/0 M(config-if)# ip address 192.168.50.234 255.255.255.252 B2(config)# int fa0/0 B2(config-if)# ip address 192.168.50.33 255.255.255.224 B2(config)# int se0/1 B2(config-if)# ip address 192.168.50.230 255.255.255.252 B2(config)# int se0/0 B2(config-if)# ip address 192.168.50.226 255.255.255.252 B1(config)# int fa0/0 B1(config-if)# ip address 192.168.50.1 255.255.255.224 B1(config)# int se0/1 B1(config-if)# ip address 192.168.50.233 255.255.255.252 B1(config)# int se0/0 B1(config-if)# ip address 192.168.50.225 255.255.255.252
All the Tred’s network internal segments are addressed and you still have 5 /27 subnets and 5 /30 subnets available within the original class C range for future growth.
Challenge Question: If a single summary route describing all Tred’s networks was to be sent to Tred’s ISP, which would it be? Answer: The summary route would be 192.168.50.0/24 which means any address within this range would be forwarded to router M by ISP.
Note: There are other ways to optimize addressing scheme on point-to-point links which were not mentioned on this case study. The ip unnumbered feature available on Cisco routers is an example. For more information on ip unnumbered addressing and configuration consult: http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094e8d.shtml
