While most companies that make Cable/DSL routers call them routers, in
all actuality, they aren't. The functions they perform are similar to
actual routers, and some models might even have the ability to be used
as an actual router, but the most common use of the Cable/DSL router
isn't as a router. It's actually a
gateway.
Most
Cable/DSL routers use a process called network address translation (NAT)
to act as a gateway for multiple computers to the internet. This is how
you are able to run a private network, and have it only use one IP
address from your service provider. NAT is, by its nature, secure, which
is why most Cable/DSL routers also claim they have a
firewall as well.
We'll get into the security/firewall bit in just a moment.
First, I want to explain how NAT works. When you turn on your computer,
it gets a network (IP) address from the
DHCP server, a process in your
router that provides network addresses to computers that request them.
This is usually an address that is not possible to be on the internet,
as it is reserved for private network use. Usually, it will be something
like 192.168.1.100, or something similar.
Now,
when you use your computer to visit a web site, your computer makes a
connection to the router telling it where it wants to go. The router
then makes the connection to the server. Since your computer has a
private network address, any reference to it is translated by the
router to reflect the public internet address your router has, so that
the server can get the data back to the router. The router starts
receiving the data and passes it back to your computer.
So, how do
these devices also act as firewalls, keeping your home network secure?
Well, since there is only one internet address that is in use by your
router, your router has to know where to send an incoming request.
Unless you have set up a port forward to a specific computer, your
router won't know where to send anything, and will just toss it in the
digital wastebasket.
Cable/DSL
Router
Most
Cable/DSL routers use a process called network address translation (NAT)
to act as a gateway for multiple computers to the internet. This is how
you are able to run a private network, and have it only use one IP
address from your service provider. NAT is, by its nature, secure, which
is why most Cable/DSL routers also claim they have a
Now,
when you use your computer to visit a web site, your computer makes a
connection to the router telling it where it wants to go. The router
then makes the connection to the server. Since your computer has a
private network address, any reference to it is translated by the
router to reflect the public internet address your router has, so that
the server can get the data back to the router. The router starts
receiving the data and passes it back to your computer. 
So, how do
these devices also act as firewalls, keeping your home network secure?
Well, since there is only one internet address that is in use by your
router, your router has to know where to send an incoming request.
Unless you have set up a port forward to a specific computer, your
router won't know where to send anything, and will just toss it in the
digital wastebasket.