How To Configure Cisco Router to Work With Cable Internet
If you just bought your first used Cisco router, I bet one thing you want to immediately do is connecting the Cisco router to the cable internet.
I know I did, I have cable internet for my home network. Previously I used Linksys router to get my LAN connected to the internet.
Configuring Cisco router to connect to the cable internet is easy work, even if you know only the basic configuration stuff in Cisco.
Configure IP Address of the Router’s Interfaces
The router will get the ip configuration from DHCP server of the ISP, the Ethernet 0/0 port I use as the exit point to the internet.
The Ethernet 0/1 will be the port where my computer is connected. I’m going to set private IP address as the gateway for the computer.
Configure Routing
This example only shows the basic static routing, the router will send all request from the client (from port Ethernet 0/1) to the port Ethernet 0/0.
We need to first set the IP address of the router’s interfaces to begin configuring Cisco router to work with cable internet.
If you configure the router for the first time, connect to it using the console cable.
the interface ethernet 0/0 is connected to the cable modem and interface ethernet 0/1 connected to my PC.
Ethernet 0/0 is using configuration got from the ISP so we’ll set it to receive IP address from ISP’s DHCP server. Always remember to give no shutdown command on the interface:
router> enable
router# configure terminal
router (config) # interface ethernet 0/0
router (config-if)# ip address dhcp
router (config-if)# no shutdown
Now to set the Ethernet 0/1 port as the picture above, we can just jump right to the interface 0/1 configuration mode:
router (config-if)# interface ethernet 0/1
router (config-if)# ip address 192.168.1.1 255.255.255.0
router (config-if)# no shutdown
You have successfully configure IP addresses to your interfaces, you can check it using the following command:
router# show ip interface brief
Interface IP-Address OK? Method Status Protocol
Ethernet0 xxx.xxx.xxx.xxx YES DHCP up up
Ethernet1 192.168.1.1 YES NVRAM up up
The show ip interface brief is a very useful command, you would want to use it to check the status of your interfaces.
The interface column shows you all the interfaces you have, the IP-Address is of course shows the addresses of the respective interfaces.
The Method column shows whether the addresses given by a DHCP server or you configured it yourself (stored in NVRAM) or it can also shows TFTP – configuration from TFTP server.
When you finished this configuration, your router will be receiving IP address on interface 0/0 from DHCP server of the ISP, and the interface 0/1 will be ready to communicate with network 192.168.1.0
Setting Cisco Router as DHCP Server
This option really is optional if you want to set Cisco router to work with cable internet, but this is a good chance to add your skill in configuring Cisco devices.
Now it’s time to configure your router as DHCP server.
To set a DHCP server, you will configure a pool of network IP addresses that you want to give out to the clients (PC, printer, NAS, etc).
I want to give out the IP addresses from the network 192.168.1.0.
First thing you need to configure is to exclude the IP addresses that you dont want to give out.
For example, I’ve configured the router interface 0/1 to be 192.168.1.1, then I need to exclude 192.168.1.1 so the router won’t give out this address.
You can configure the exclusion in the router’s global configuration mode:
router> enable
router# configure terminal
router (config)# ip dhcp excluded-address 192.168.1.1
This command is very useful especially if you need to exclude a range of IP addresses, if you need to exclude say 192.168.1.1 until 192.168.1.10 you can do it like this:
router (config)# ip dhcp excluded-address 192.168.1.1 192.168.1.10
After the ip dhcp excluded-address we give the low IP address and the high IP address, this way your router not give IP addresses from 192.168.1.1 to 192.168.1.10, the router will start giving out address from 192.168.1.11 and so on.
Next thing to do is creating a pool of addresses, when issuing the ip dhcp pool, you’ll be taken to the dhcp configuration mode where you can set the additional parameters beside the ip address and subnet mask to the clients.
In this example I want to make a pool with the name of HOME_CLIENTS
router> enable
router# configure terminal
router (config)# ip dhcp pool HOME_CLIENTS
router (dhcp-config)# network 192.168.1.0 255.255.255.0
router (dhcp-config)# default-router 192.168.1.1
router (dhcp-config)# import all
At the third line above you can see the configuration command of a pool with the name HOME_CLIENTS.
Fourth line shows that the pool HOME_CLIENTS will give out the addresses in the network 192.168.1.0, with the exception of the addresses in the ip dhcp excluded-address 192.168.1.1 that I showed you previously.
Fifth line tells the clients should be given a default gateway address of 192.168.1.1 (the router’s IP address).
The sixth line is the one important thing for configuring DHCP server in cable internet environment.
The import all command tells the router to give out other configuration received from the ISP cable internet DHCP server to the clients in the LAN.
For example, most ISP will give the DNS servers IP addresses from their DHCP server and this configuration might change depends on the ISP, so you definitely want to give this configuration out to the clients.
If you have your own DNS server in the LAN, you can tell the clients to use this DNS server using the following command:
router (dhcp-config)# dns-server 192.168.1.2 192.168.1.3
The above command will send out DNS server address of 192.168.1.2 and 192.168.1.3 to the clients.
You can also configure the router to use the above DNS server using the following command in the global configuration mode:
router (config)# ip name-server 192.168.1.2 192.168.1.3
When connecting your router to the cable internet through cable modem, your router will receive a dynamic IP address from the ISP DHCP server according to the scenario.
While you only get one IP address from the ISP, you also need to connect more than one computer to the internet.
Plus the public IP address is different network with the private IP addresses in your LAN.
NAT can solve this problem, it stores the requesting private IP addresses in the address translation table of the router, translates every request from your LAN and forward it to the internet using the single public IP address.
Now NAT can be used in different scenario, but I’ll save them for future posts, for now the NAT form that we’ll use is many-to-one scenario of NAT.
Many private IP addresses translated to be one public IP address, some people call it overloading and/or Port Address Translation (PAT).
NAT Overload will assign a unique logical port number to every request from the LAN to the internet thus PAT.
For example, if you have a public IP address of 202.1.1.1, then for a request from the user of 192.168.1.20 in your LAN will be translated into 192.168.1.20:1720 for the incoming request tothe router, and 202.1.1.1:1521.
This is how the router can identify which request goes to which device.
Create Access List
router> enable
router# configure terminal
router (config)# access-list 101 permit ip 192.168.1.0 0.0.0.255 any
Issue PAT command
router (config)# ip nat inside source list 101 interface Ethernet0/0 overload
Identify interfaces for ip nat inside and outside
router (config)# interface ethernet0/0
router (config-if)# ip nat outside
router (config-if)# interface ethernet0/1
router (config-if)# ip nat inside
That’s it three easy to remember steps for configuring NAT/PAT, one last thing to do for connecting your router to the cable internet is configuring default route.
Configuring Default Route
I decided to include configuring default route into this post since I only have a simple network topology and we only need one line of command to configure the default route.
Here’s how we do it, from the last command we jump back to the global configuration mode:
router (config-if)# exit
router (config)# ip route 0.0.0.0 0.0.0.0 ethernet0/0
What the above command does is to route all request that point to any ip address that the router doesn’t know (0.0.0.0 0.0.0.0) to the ethernet0/0 interface.
If you have a static public IP address from the ISP then you can replace the ethernet0/0 with the IP address given by the ISP.
This is a REPOST thanks to the Networking Newbie site
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