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IP Routing Fundamentals

From: IP Routing Fundamentals
Author: Mark Sportack
Publisher: Cisco Press (53)
More Information

WAN Gateway

Perhaps the most familiar, and important, of a router's LAN-level functions is its use as a gateway to the WAN. This is the function that LAN-based routers were originally designed for. In this original context, a LAN's router was its gateway to the internetwork that lay beyond its border. The router can function in this capacity because it supports operation at Layer 3, the network layer, of the OSI reference model.

The network layer encompasses all the protocols, address architectures, and other mechanisms needed to pass data beyond the domain boundaries of LANs. To utilize network layer protocols requires the use of a networking protocol, such as IP. Networking protocols, known more specifically as either routed or routable protocols, each have their own addressing schemes. These addresses must be used by any/all devices that need to communicate beyond the LAN's boundaries. Communications within a LAN's boundaries can be affected through data link layer addressing.

Implementing IP addresses, as well as other internetworking protocols, creates a new domain: a Layer 3 domain. In the case of the IP protocol, this new domain is the IP network domain.

IP Network Domains

Much like a MAC broadcast domain defines the boundaries of a LAN, IP also defines its borders. All devices within a given IP network domain must have the same IP network address. They have unique host numbers that, when appended to the network address, yield a unique address that can be used across network domains. For example, an IP network address might be The 192.168.9 portion of this address is the network's numeric address. The last digit (0) identifies a specific host within that network number. Zero is a reserved value, which cannot be assigned to a host. It always identifies a network's number. Hosts addresses can range from 1 to 255, for a total of 254 usable addresses in this particular type of IP address. It is important to remember that the IP boundaries described in this section are applicable regardless of which type of IP addressing is used. Subnets, VLSM, classful addresses, and even IPv6 addressed networks all conform to this description of the IP boundaries.


For a more detailed examination of the two IP protocols, refer to Chapter 5, “Internet Protocols Versions.”

Figure 3-16 illustrates a typical LAN that is using a router as a gateway to an IP WAN. In this example, every device in the LAN has its own IP address, and they are all part of the network.

Figure 3-16. Using a router as an IP WAN gateway.

As indicated in Figure 3-16, the LAN's IP network domain extends up to, and includes, the router interface port. The domain does not include the entire router or all its ports. Each port can be programmed to function as the interface for a different IP network domain. This is one of the attributes that make a router so uniquely capable of internetworking. It can function as a point of intersection for two or more different networks. More importantly, it can support the physical intersection of networks (either LANs or connections to other routers) with a series of services that facilitate forwarding of data between different networks.

Using the Gateway

As explained in Chapter 1, “An Introduction to Internetworking,” LAN-attached devices transmit framed data. The payloads of these frames, more likely than not, contain network layer packets. Therefore, users of LAN-attached devices access the WAN gateway via transmitted frames.

The gateway router enables devices to communicate and share data with devices that lie beyond their LAN's boundaries. This requires only the use of a routed protocol suite, such as Transmission Control Protocol/Internet Protocol (TCP/IP) or Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX), and proper use of that protocol suite's addressing mechanisms. Transmitting devices (better described as originating devices) must specify the full network (for example, IP) address, including the network and host numbers as the destination. This destination address is recorded in the header of each IP packet transmitted, along with other pieces of information. IP packets are placed in the framing required by the LAN that the device is connected to and transmitted over the LAN.


Devices that initiate, or originate, a communications session are properly identified as originating, or source, devices rather than transmitting devices. The reason for the distinction is simple: After the communications session is established, data may flow in both directions. Therefore, differentiating between a transmitting and a receiving device becomes a per-packet endeavor. However, the source and destination of any given communications session remain constant for the duration of that session.

LAN frames, too, contain header information such as source and destination addressing. These addresses are the data link layer's MAC addresses. In the case of the LAN's gateway router, the MAC address corresponds to the router interface port's MAC address. This port accepts the transmitted frames, strips off the framing, and examines the network address of any embedded packet(s). This network address determines where the router forwards the packet. This forwarding process is examined in more detail in Chapter 7, “The Mechanics of Routing Protocols.”


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