Section 13.5. Cable Modems

13.5. Cable Modems

When one wants Internet access enough, any wire will do. There are intensive experiments to use even the electricity grid to deliver Internet access. So it's not surprising that companies that built their business plans on delivering laser-crisp pictures of sporting events (the cable TV firms) would realize they could devote one of their channels to a local area network carrying digital datahence the advent of cable Internet access.

The bandwidth on the cable infrastucture theoretically ranges up to 10 Mb (like an old Ethernet on coaxial cable), but some providers today achieve more throughput. Usually, they span a web of nodes, each of which offers 10 Mb or less. The neighbors who share the node have to share the bandwidth. So if there are two users on the node, each gets 5 Mb. Furthermore, some customers near the center (the central server, which is usually connected via the optical fiber cable to the rest of the Internet) have more bandwidth than those at the periphery. So check your cable network very carefully before you sign up for Internet access. Talk to the technical support first, and if they cannot answer your question, this might be a good indication about how good their support is going to be later.

ISPs that sell clients Internet access through cable modems will usually take responsibility for service all the way to your Ethernet card. They will give you a cable modem with a coaxial cable connected to their cable infrastructure, and an Ethernet (RJ45) connector on your side. To set up your Internet connection, you will need to know the IP address assigned to the cable modem, the network mask, and the gateway; your ISP should provide you with this information together with the cable modem. On your side, you need only start up your (carefully configured) Ethernet card with the data provided to you by your ISP:

     /sbin/ifconfig eth0 IP_address netmask

Next, tell the kernel about the gateway:

     /sbin/route add default gw gateway meTRic 1

This is the setup for a standalone Linux workstation. If you plan to run a small network behind the Linux machine, you will have to use masquerading, as described earlier in this chapter; you can find help for this in a book that covers Linux firewalling, such as the Linux Network Administrator's Guide and the Linux iptables Pocket Reference (O'Reilly). Some Linux distributions, such as Slackware, turn off IP forwarding by default, which means that masquerading will not work. If this is the case, add the following line to your startup script:

     echo 1 >/proc/sys/net/ipv4/ip_forward

All cable modems can be configured remotely. If you are unlucky and have an ISP that does not configure the cable modem for you, you have to configure it from scratch; this will require more than average knowledge about how TCP/IP works, and you should probably seek assistance from your ISP (or switch to one that does configure the modem for you).

In some cases, the cable modem is configured such that it is works only with one particular Ethernet card, and you have to give the MAC address of your card to your ISP for configuration purposes. If this is the case, you need to ask your ISP to reconfigure the modem in case you should switch Ethernet cards (or computers).