IP/Network

ICS(Internet Connection Sharing)

ICS is a mechanism by which you can 'share' a Network port which has access to public internet with other network port which does not have access to internet. You may ask why we need this kind of cumbersome concept if you are so used to getting access through WiFi Access at home or office. However, this was one of the popular way for many people to get access to internet via Wireline LAN when you purchased only one IP (public IP) for internet access.

Even now there are some cases where you still need this kind of mechanism for various reason.

NOTE : When I was first trying to understand on the concept of ICS, I was so confused with ICS and NAT(Network Address Translation). If you are also get confused about these two concept, you may think of this like this : There are multiple technologies that make ICS possible and NAT is a fundamental technical component of ICS. If you search the ICS (for example Ref [1],[2]), you would notice that NAT and DHCP are the most important technical component of ICS (especially when we are talking about ICS on Windows). So if you are familiar with the concept of NAT and DHCP, you would easily understand how ICS works. For NAT, I would recommend you to watch the video (Ref [3],[4]). For DHCP, refer to my note on DHCP.

One of the simplest configuration for ICS may look as follows.

You have two PC in the illustration : PC(1) and PC(2).  PC(1) has two network cards : NIC (A) and NIC (B). PC(2) has only one network card : NIC (C).

You have purchased only one IP from your ISP (Internet Service Provider) and allocate that to one network card (NIC (A)) of PC(1).

Now you want to use PC(2) to get access to public internet, but you cannot have direct access to the internet since you haven't purchased IP for that PC.

In this case, you can connect PC(2) to PC(1) as shown below and enable ICS. Then both PC (1) and PC (2) can have access to public internet. (The procedure to enable ICS in Windows may vary a little depending on Windows version. You can easily google about the procedure for the specific operating system that you are using).

If you are using a hub, you can get multiple PC to be able to share PC(1)-NIC(A) to get access to public internet as shown below.

Main purpose of this page is not for showing how to configure ICS on your PC since you can easily find tutorials by Googling. My intention is to describe how this ICS works internally. Following example shows how ICS works. Simply speaking, ICS is a kind of NAT (Network Address Translation)

< Example >

Reference :

[1] Internet Connection Sharing (Wikipedia)

[2] ICS – Internet Connection Sharing (webopedia)

[3] NAT basics for beginners CCNA - Part 1 (YouTube)

[4] NAT basics for beginners CCNA - Part 2 (YouTube)