Daryl's TCP/IP Primer

Addressing and Subnetting on the Near Side of the 'Net

[<--Prev] 1. Overview and Scope 2. Intro to Ethernet 3. The Bottom of the OSI Model 4. Why is IP so much more difficult than IPX? 5. IP Addresses, Subnet Masks, and Subnetting 6. Subnetting, Bit by Bit 7. Daryl's Subnet Calculator 8. Routing and Static Routes 9. Troubleshooting 10. TCP and UDP Communication 11. Network Address Translation (NAT) 12. The Domain Name System (DNS) 13. Tips for Building an IP LAN 14. Packet Analysis 15. WAN Connectivity 16. Questions and Answers 17. Update Notifications/Comment Form 18. Other Sources 19. Glossary [Next-->] [Printable Version] [Daryl's ColdFusion Primer] [About Daryl]

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8. Routing and Static Routes

Before: Network 192.168.1.0:

After: Split into three parts using a subnet mask of 255.255.255.192

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What we need to do now is tell the router what happened...

First, you have to tell the old router that the network attached to its Ethernet interface has changed (specifically, the network mask has changed, and often, the address of the Ethernet interface has changed.) If you were adding a new subnet, rather than splitting an existing one, then you could probably skip this step.

Second, you have to tell the old router where to find the new network (what the next hop is.) A typical command would look something like this:

ROUTE 192.168.1.64/255.255.255.192 192.168.1.2

What you're telling the old router with that statement is, "if you need to route packets to the subnetwork that starts at 192.168.1.64 and has a subnet mask of 255.255.255.192, you should forward all packets intended for that network to the router at 192.168.1.2."

Third, be sure the default route for the new router is set to 192.168.1.1.

Note that the automatic routing protocol (IP) RIP does not understand subnet masking. If you are using protocols that do, such as OSPF or EIGRP, then you probably aren't reading this document. Actually using routing protocols tends to be irrelevant on the "near side" of the net, since there is generally only one path to the Internet from any given workstation on a LAN. Multiple routes tend to be a problem only closer to the backbone, and that's your ISP's problem.

Next: Troubleshooting