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Chapter 2: Networking
address by various means, including a DHCP server, by you directly entering it, or sometimes by a
PC generating one of its own.
Ethernet requires that each TCP/IP stack on the Wireless Cable Gateway also have associated with
it an Ethernet MAC (Media Access Control) address. MAC addresses are permanently fixed into
network devices at the time of their manufacture. 00:90:64:12:B1:91 is an example of a MAC
address.
Data packets enter and exit a device through one of its network interfaces. The gateway offers
Ethernet, USB, HPNA, and 802.11b wireless network interfaces on the LAN side and the DOCSIS
network interface on the WAN side.
When a packet enters a network interface, it is offered to all the TCP/IP stacks associated with the
device side from which it entered. But only one stack can accept it — a stack whose configured
Ethernet address matches the Ethernet destination address inside the packet. Furthermore, at a
packet's final destination, its destination IP address must also match the IP address of the stack.
Each packet that enters a device contains source MAC and IP addresses telling where it came
from, and destination MAC and IP addresses telling where it is going to. In addition, the packet
contains all or part of a message destined for some application that is running on the destination
device. IRC used in an Internet instant messaging program, HTTP used by a web browser, and FTP
used by a file transfer program are all examples of applications. Inside the packet, these
applications are designated by their port number. Port 80, the standard HTTP port, is an example
of a port number.
The Networking section of the router performs many elegant functions by recognizing different
packet types based upon their contents, such as source and destination MAC address, IP address,
and ports.
Three Networking Modes
Your gateway can be configured to provide connectivity between your cable company and your
home LAN in any one of three Networking Modes: CM, RG, and CH. This mode setting is under the
control of your cable company, who can select the mode to match the level of home networking
support for which you have subscribed. All units ship from the factory set for the RG mode, but a
configuration file which the cable company sends the cable modem section during its
initialization can change it.
Chapter 2
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