Reference Manual

Introduction

ICS Modules (also known as DSL Routers, Cable Modem Routers and ISDN Routers) are small, stand-alone modules about the size of an Ethernet Hub. They are used to connect a small number of  machines on a private LAN to the Internet via a Dial-Up Networking connection, an ISDN connection or similar. They work very much like software IP routers (e.g. NAT32) and perform Network Address Translation and TCP/UDP Port Mapping. They have the advantage that they are small, low-power devices which can be left switched on and permanently connected to the Internet. Some even support Wireless LAN access for your local machines. The Apple Airport is an example.

Hardware

ICS Modules usually consist of a single-chip embedded processsor with a built-in serial port and one or two LAN interfaces. The CPU is often just a small 25MHz unit, and RAM is often limited to only about 64K bytes. While such CPUs perform adequately for DUN connections, they are probably underpowered for ADSL and Cable Modem connections.

Software

The software controlling the module is stored in Read Only Memory, although the more expensive models may store software in FLASH EPROM. Such models can be upgraded, but only if the vendor makes such upgrades available. Most vendors do not upgrade their software very frequently, and this can be a problem in the rapidly evolving world of networking.

Also, re-programming FLASH memory is a critical task, and if the procedure goes wrong, you may be left with a broken box.

Limitations

The crucial disadvantage of such boxes is that they use the registered IP address entirely for themselves. In other words, none of your machines has full, registered IP address access to the Internet. All your machines have Address Translated access only, meaning that many of the typical "NAT-hostile" applications such as NetMeeting and other H.323-based applications will give trouble.

With NAT32, the machine on which it runs has unmodified access to the Internet, and all applications on that machine continue to run exactly as before.

A second problem is the limited number of private machines (often only 8 or so) which can share the connection. In addition, most ICS modules will not allow machines on networks behind a gateway on your private LAN to share the connection at all. This will rarely be a problem for home users, but it may become a problem for small business users, particularly as the business expands.

Finally, ICS Modules are often built for one network type only. If you start out with a module for a DUN connection, and later decide to upgrade to DSL, you may need to purchase a new ICS Module for that purpose.