Internetwork Routing

The following terms are essential to your understanding of routing:

End Systems . As defined by the International Standards Organization (ISO), end systems are network devices without the ability to forward packets between portions of a network. End systems are also known as hosts.

Intermediate Systems . Network devices with the ability to forward packets between portions of a network. Bridges, switches, and routers are examples of intermediate systems.

Network . A portion of the networking infrastructure (encompassing repeaters, hubs, and bridges/Layer 2 switches that is bound by a network layer intermediate system and is associated with the same network layer address.

Router . A network layer intermediate system used to connect networks together based on a common network layer protocol.

Hardware Router . A router that performs routing as a dedicated function and has specific hardware designed and optimized for routing.

Software Router . A router that is not dedicated to performing routing but performs routing as one of multiple processes running on the router computer. The Windows 2000 Server Router Service is a software router.

Internetwork . At least two networks connected using routers. Figure 1.1 illustrates an internetwork.

Figure 1.1 An Internetwork

Addressing in an Internetwork

The following internetwork addressing terms are also important to your understanding of routing:

Network address . Also known as a network ID. The number assigned to a single network in an internetwork. Network addresses are used by hosts and routers when routing a packet from a source to a destination in an internetwork.

Host address . Also known as a host ID or a node ID. Can either be the host's physical address (the address of the network interface card) or an administratively assigned address that uniquely identifies the host on its network.

Internetwork address . The combination of the network address and the host address; it uniquely identifies a host on an internetwork. An IP address that contains a network ID and a host ID is an internetwork address.

For detailed information about how IP implements network ID and host ID addressing, see "Introduction to TCP/IP" in the Microsoft ® Windows ®  2000 Server Resource Kit TCP/IP Core Networking Guide .

When a packet is sent from a source host to a destination host on an internetwork, the Network layer header of the packet contains:

• The Source Internetwork Address, which contains a source network address and source host address.

• The Destination Internetwork Address, which contains a destination network address and destination host address.

• A Hop Count, which either starts at zero and increases numerically for each router crossed to a maximum value, or starts at a maximum value and decreases numerically to zero for each router crossed. The hop count is used to prevent the packet from endlessly circulating on the internetwork.