Sams Teach Yourself TCP/IP in 24 Hours, Fourth Edition

Routing in TCP/IP

In its most basic form, a router is a device that filters traffic by logical address. A classic network router operates at the Internet layer (OSI Network layer) using IP addressing information in the Internet layer header. In OSI shorthand, the Network layer is also known as Layer 3, and a router is sometimes called a Layer 3 device. In recent years, hardware vendors have developed routers that operate at higher layers of the OSI stack. You learn about Layer 4–7 routers later in this hour, but for now, think of a router as a device that is operating at the Internet layer or OSI Layer 3—the same level as IP addressing.

Routers are an essential part of any large TCP/IP network. Without routers the Internet could not function. ...