S/390 PartnerWorld for Developers, ITSO/EFS Project EFS Systems on a Linux Base: Additional Topics

Chapter 3. Networking 35

3.6.2 One solution

We purchased a Netgear Cable/DSL ProSafe Firewall/Print Server Model FR114P unit. The

unit cost approximately US$110 at the time of writing, although the prices of such units are

dropping rapidly. This is a small router that can be used with 10/100 Mbps Ethernet LANs.

The

Cable/DSL portion of the product name can be misleading. Cable and DSL “modems”

provide an Ethernet connection

26

as the computer interface. This interface can be used as a

normal Ethernet connection to a PC.

27

This particular unit allows 10 or 100 Mbps connections

to the external LAN (which might be a cable or DSL modem). Other similar units are available

from other vendors, but this was the only one we found (at the time of writing) that provided

100 Mbps connections to the external LAN. All the Netgear documentation refers to the

external LAN as the WAN. The unit has one WAN Ethernet connector and four local LAN

Ethernet connectors, as shown in Figure 3-3.

Figure 3-3 Small router overview

It is important to understand the terminology of the router. In this terminology, the WAN is the

external network and the LAN is the local network created by the router. In the situation we

describe, the WAN is probably a corporate LAN (but it could be a DSL or cable modem). In

this chapter, we use the term

external LAN instead of WA N, and local LAN instead of LAN.

The key element of the solution is the Network Address Translation (NAT) provided by the

router. It automatically translates the local LAN addresses (the three addresses shown in

Figure 3-3 in the 192.168.0.x range) to the single external IP address (shown as 9.12.6.140 in

the figure). The external world sees only a single system at 9.12.6.140. The router maintains

state information that keeps track of which local system (for example, z/OS at 192.168.0.111)

25

This statement assumes the cable or DSL interface does not provide a Network Address Translation (NAT)

function. The situation is more complex when NAT is provided by the “modem” and we do not address this in this

discussion.

26

These are all RJ-45 connectors.

27

This usage is not documented by Netgear and may not be supported by them. We can only state that we have

used it as described here and it works quite well.

WAN Connection

LAN Connections

Linux:

192.168.0.110

z/OS

192.168.0.111

Windows:

DHCP client

(e.g., 192.168.0.3)

ThinkPad

Desktop

Ethernet (corporate LAN)

or Cable Modem

or DSL line

Fixed IP address

or DHCP assigned

(9.12.6.140)

Router

192.168.0.1

Two IP addresses

using one physical

port