author spends the first page describing the basic purpose of a router:
to connect two or more networks. He provides us with a small photo of
ports on a commercial router (a Cisco 2611, which went out of
production in 2003). He points out a difference between home routers
and commercial routers: home routers typically only connect two
networks (yours and your ISP's) but they typically incorporate a
switch, which commercial routers do not. Think about it like this. A switch is a networking device: it allows hosts to connect to your network. A router is an internetworking device: is allows your network to connect to another network.
As we have discussed before, the home router also typically includes
software to act as a DHCP server, and my include firewall software.
The text and the LabSim videos both discuss the idea that a router receives a frame from its local network, unwraps the frame and examines the Network layer information to determine whether this message has to stay on this network or be passed on to another one. We discussed this last week. The router clears the MAC addresses from the frame, reads the information in the IP addresses, writes what needs to be written in the frame, and passes the frame to another router, or a switch if the network address is local.
The text continues with a discussion of routing tables. A router keeps a table that is really a set of rules. It says what to do with incoming packets, based on the network addresses in those packets. The rules in this table can be set set automatically, set by an administrator, set by a protocol, or can be set by all three methods. Each line has a rule that describes some kind of packets, and a route to use for packets that are described by that rule. The example on page 185 is a bit small to read clearly. It is meant to explain a routing table used by a home router. As such, it only references two routes on three rules. The table shown in the text has three lines and four columns. It looks like this:
To understand the table you need to know several things:
The first two rules are placed in the table automatically when those two networks are detected. They essentially say to pass along any message for hosts on those networks on those networks. No other router is mentioned in the Gateway column for those rules. There is no next hop, because the message is already being handled by a router on the correct network. On the third rule, a catch all filter is used: for a host on any network, the next hop is my ISP's gateway, which is on the network connected to my WAN port. This is the router's default rule: unless I have already told you otherwise, do this with any packet you see.
The text explains that all hosts on an IP network keep routing tables as well. We see how to display the routing table of a Windows XP PC on page 187. In another page or so, the text finally mentions how to see the routing table on most devices:
There are several differences in the Windows table we are shown. The first is that the default route is shown as the rule at the top of the table instead of the bottom. This is because Windows routing tables are evaluated from the bottom up, instead of from the top down.
Some columns are different as well. The first four columns are called by some different names, but they mean the same thing: the network the message is sent to, the subnet mask for that network, the gateway (if any) to hand it to, and the port that is used to reach that gateway. The fifth column shows metrics. These are values the device will use to choose a route whenever it has two or more active routes to the same network. A route is active if there are live devices up and running, for every hop to destination. If a route goes down, it is good to have an alternate route available.
In the Interface column of the Windows routing table, we see only two values: one is the IP address of the only NIC in this machine, and the other is the local loopback address. When a route leads to the local loopback, the signal does not need to leave the device. When it leads to the NIC, it does.
For the routes that lead to the NIC, there are two possible values in the Gateway column. When the signal is handed off to a device on the local LAN, the address of our NIC appears in the Gateway column. (If the PC was running Windows Vista or later, this would say on-link instead of the NIC's IP address.) When the signal is going outside our LAN, the address of the router for the LAN (default Gateway) is in the Gateway column.
On page 190, the text shows us a portion of a Cisco router's routing table. Note that when the command is given to show IP routes, the first several lines of output are a list of codes used to tell the reader how the router learned about that route. There are only three lines in the actual table in this example. Two routes were learned by direct connection (code C), and one was a static route set by an administrator (code S). Most of the other codes refer to routing protocols that might be used by one router to request or to tell another router about routes.
The text spends a great many words telling you that all the possible IPv4 addresses have been assigned. Even so, it tells you that once upon a time, if you were setting up a network you would make an application to IANA to get some. Now, it is easier: you set up a private address network, and use Network Address Translation to connect to the Internet through your ISP. The text explains variants of NAT:
The text turns to a discussion of dynamic routing on page 196.In this context, dynamic means anything other than the static routes that are learned from local connections and from routes entered by administrators. Some familiar concepts are offered up again. On page 198, the text discusses several different ways to calculate a metric for a route:
A dynamic protocol is usually one of two types:
Some protocols are best used for specific purposes.
The text mentions console cables on page 210, and shows an example of a Yost cable, which has a serial connector on one end and an RJ-45 on the other. We have also talked about rollover cables, which are used for the same purpose: connecting from a host to the console port on a router. You do that to sign in as an administrator and configure your router. You use a terminal emulation program on the host to make the connection to the router. Two terminal emulators are mentioned in the text: PuTTY and HyperTerminal. A few pages later, the text also mentions Secure Shell. The settings for the terminal emulator are listed on page 210 as well. They are like the settings you use in a modem program:
The text continues to introduce you to Cisco IOS, the operating system used on commercial Cisco equipment. The author observes that you need to know a lot about the Cisco IOS for the Cisco certifications, but not so much for the Network+ certification.
There are other ways to configure a router. If you own a home router from Linksys, you own a Cisco router that does not have a console port. You use a web interface to configure it instead, and you can do the same thing with their larger routers. However, there is a warning on page 212 that you should consider. The text tells us never to plug a new router into an existing network. That's a little strong. I think the author means that we should directly connect one host to the router and configure it first before making it a physical part of our network. Teach it about our rules first, then install it in its intended place.
The text continues with a larger concept. If we only have one router to worry about, the methods above are fine. If we have dozens to manage, we are better off using some kind of Network Management Software. Note the example screen shots on page 213 and 214 of network management tools. Vendor tools are recommended for a larger feature set.
On page 215, the text discusses basic setup of a router, and mentions that you will typically connect the WAN port of the router to your ISP's connection device (such as a cable modem) and will typically use DHCP to obtain an address for the router that fits on the ISP's network.
You will want to set up your LAN on the router as well. If you use the router to assign addresses on your network, decide whether you will use the router's default address scheme. You will want to change that if you already have an address scheme that you must match, which is often the case on a business network.