Chapter 3: Networking
Illustrations contained in this document are for representation only.
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can use to configure its networking side; when you communicate with the networking
side, your communication is following this path.
Each packet on the Internet addressed to a PC in your home travels from the Internet down-
stream on the cable company’s system to the WAN side of your Wireless Cable Gateway. There
it enters the Cable Modem section, which inspects the packet, and, based on the results,
proceeds to either forward or block the packet from proceeding on to the Networking section.
Similarly, the Networking section then decides whether to forward or block the packet from
proceeding on to your PC. Communication from your home device to an Internet device works
similarly, but in reverse, with the packet traveling upstream on the cable system.
Cable Modem (CM) Section
The cable modem (or CM) section of your gateway uses DOCSIS Standard cable modem
technology. DOCSIS specifies that TCP/IP over Ethernet style data communication be used
between the WAN interface of your cable modem and your cable company.
A DOCSIS modem, when connected to a Cable System equipped to support such modems,
performs a fully automated initialization process that requires no user intervention. Part of
this initialization configures the cable modem with a CM IP (Cable Modem Internet Protocol)
address, as shown in Figure 3, so the cable company can communicate directly with the CM
itself.
Networking Section
The Networking section of your gateway also uses TCP/IP (Transmission Control Protocol/
Internet Protocol) for the PCs you connected on the LAN side. TCP/IP is a networking protocol
that provides communication across interconnected networks, between computers with
diverse hardware architectures and various operating systems.
TCP/IP requires that each communicating device be configured with one or more TCP/IP
stacks, as illustrated by Figure 4. On a PC, you often use software that came with the PC or its
network interface (if you purchased a network interface card separately) to perform this
configuration. To communicate with the Internet, the stack must also be assigned an IP
(Internet Protocol) address. 192.168.100.1 is an example of an IP address. A TCP/IP stack can
be configured to get this IP 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, and 802.11b/g 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