How do i map external ip's to internal ip's behind dsl router?

Posted on 2003-03-05
Medium Priority
Last Modified: 2008-02-01
I want some of the PC's on my LAN to be accesible from the internet.  I have a DSL connection and will be soon getting some public IP's from my ISP.  When I buy a router, what feature will i have to look for that will allow it to map external to internal ip's.

Help would be greatly appreciated.

(i only have 35 pts left, will increase it when another one of my questions are deleted and then accept the answer)
Question by:david_w__uk
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Expert Comment

ID: 8076047
port forwarding... basically all the router has the features:

check this:



Expert Comment

ID: 8077827
You will need to buy a multi-port hub. Forwarding port to hub then connect the nodes needing forwarding to the respective hub ports and internal nodes on the router ports without forwarding.  Or just put hub infront of the router directly connected to your access assuming your not running fiber or something needing L3 switch or router type interfaces, then connect your nodes wanting public ip's to the hub, and every thing else behind the router.  
           ____>public IP nodes
           ____>router--->private network

             ___>port forwarding-->Hub-->public nodes  
             ___>private port-->Hub-->private nodes

Hubs are just cheaper than buying a router with a bunch of interfaces and it saves you some of the configuration work and upkeep...  

Expert Comment

ID: 8077870
i think it is worth to get a router, better security and more functionalities than a hub... performance is a little faster, reliable on transfer data interms of its technology and architecture.

$40 USD man... it is not that expensive.
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Expert Comment

ID: 8078766
i agree with smallbee. dsl routers are so cheap now, and almost all of them come with firewall capabilities.

here's a link to a bunch of dsl/cable routers for you to look at.


Author Comment

ID: 8081053
My ADSL comes through my phone line, therefore i cant do it that way, i currently have a DSL Router with a built in modem, so would i need to set up static routing?  How do i let the router know of my different ip's, do i needto set up a virtual circuit (as my router calls it) for each IP, and then nat each circuit to the internal machine i want the traffic to be mapped to?

i am new to routing things, so could you explain it basically?

Expert Comment

ID: 8081469
ok... I think you are trying to host a website, FTP, email, application server, am I right?

If yes,

lets start from basic here:

1) web, ftp, email, etc.. types of servers using certain "ports" to communicate with other computers(clients) port: 80, 21, (25 for smtp[outgoing email], 110 for Pop3[incoming email]) respectively, etc.

2) say when i person open a broswer and type: www.domain.com, what it basically is: ur computer sending a web (port 80) request out and look for the ip address of the web server, and if it is found, the web server to respond with a webpage through port 80 back to your computer.

3) in the middle of looking for the ip address of the web server, there are DNS severs which will translate or resolve the domain name to be an ip address (which is the ip address of the webserver)

4) if u registered a domain name, u will see some settings about DNS entries in your registrar account setting. Depends on your DSL service provider, they might offer free DNS to host a domain name for thier customer. Call your ISP. The other thing is about static ip, ask whether they provide this. The is pretty important if u wanna host a web, mail, ftp server with ur own computer. I will explain it later

Now go to back ur ISP/DSL to ur router

say u have a domain name, and it is hosted in ISP. so when ppl tpye in ur domain.com on the broswer, DNS in ISP will translate it to the ip of ur DSL modem...

a router is a hardware that can block all requests (ports), and transfer the differnt ports request to different ip address (local computers in ur LAN at home).
so ppl, can use different computers to host different purpose of server. that is call port forwarding

so what u need to set in the router is ... there should be something call port forwading, virtual servers, etc... go to that page, and u will see something like:

port          to what ip          port         TCP/UDP

1) port is the request from outside,

2) to what ip is the ip address of ur computer in ur LAN

3) port you want to use to accept the request/respond back to the computer which send the request, usually it is the same of the 1st port... for newbies, just put the same port number of the 1st one

4) TCP/UDP, usually is TCP, depends on what application u use, but web, ftp, email using TCP

so how it works... when ppl type in yourdomain.com

1) port 80 or web request was sent from the person computer

2) go throught DNS to translate or resolve to ip

3) request come in to ur router

4) your router see port 80 request, and according to the port forwading setting, it routes it to ip of one of ur computer in the LAN (assumpt u have setup the web or port 80 request forwarding)

5) your web application server in that computer pick up the port 80 request and respond with the webpage back to the person who type in urdomain.com in his/her broswer.

there are few more things about setting up a webserver (software part, e.g. IIS or apache, etc.).. but I think I explained the Hardware part pretty much in detail.

Anyhow, read the manual of ur router... u will understand more

good luck


Author Comment

ID: 8081842
How do i use it so it maps the external ip's to the internal ones (as if the internal pc's actually had the external ones) without doing it with ports.  Just a straight through IP translation.  Is it called static routing?

Expert Comment

ID: 8081928
if the internal is the same as external, and you set the computer TCP/IP setting is same as the external ip, the it lose the meaning of using a router, then ur computer is almost completely exposed to the public or internet.

you don't want to protect ur computers?

from a Linksys router,

Static routing:

If there are multiple routers installed on your network, it is necessary to configure the unit's static routing functions. The static routing function determines the path that data follows over your network before and after it passes through your router. You can use static routing to allow different IP domain users to access the Internet through this device. This is an advanced feature. Please proceed with caution.

This router is also capable of dynamic routing (see the Dynamic Routing tab). In many cases, it is better to use dynamic routing because the function will allow the router to automatically adjust to physical changes in the network's layout. In order to use static routing, the router's DHCP settings must be disabled.


Author Comment

ID: 8081982
So can I use static routing for my purpose like this?

External IP1 ---->  Internal IP
External IP2 ---->  Internal IP2

Expert Comment

ID: 8164369
You CAN do static routes as listed above.  The routers DHCP settings do NOT have to be disabled.  Assign your servers on the LAN static NAT IP addresses outside of your defined DHCP pool.  With all ports mapped from the outside this is often referred to as a DMZ or De-Militarized Zone.  This is commonly used - among other things - so that other machines on the LAN can access the same servers as those on the WAN without crossing the router and remaining on the same subnet.

Accepted Solution

smallbee earned 140 total points
ID: 8184207
find the router that support static routing... I know there is one in Linksys:


the others may have this function too...


Expert Comment

ID: 8332661


Advanced Features:
The router’s advanced features include Filters, Forwarding, Dynamic Routing, Static Routing, and DMZ Host.

GOOD luck....

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