Administrator's Guide

Chapter 3. Conﬁguring 3Com VCX IP Telecommuting Module

power of two number you selected, but under 255. In the above example, this means 0, 8,

16, 24, 32, 40, 48 and so on, up to 248. You might choose to start with 136 (17 x 8). This

would give the computers the IP addresses 196.176.1.136, 196.176.1.137, 196.176.1.138,

196.176.1.139, 196.176.1.140, 196.176.1.141, 196.176.1.142 and 196.176.1.143.

One of the IP addresses is free and can be used for an eighth computer in the future. You

must enter the ﬁrst IP address in the series, 196.176.1.136, in the Network/IP address ﬁeld.

Now you must set the mask so that only the computers with these eight IP addresses are in-

cluded in this network. Take 256 and subtract the number of IP addresses in the named net-

work. In the example, we would have 256-8 = 248. The complete mask is 255.255.255.248.

Now you have created a group of computers (IP addresses) that you can give a single name,

such as Administration.

Table of netmasks.

No. of computers Mask Bits

1 255.255.255.255 32

2 255.255.255.254 31

4 255.255.255.252 30

8 255.255.255.248 29

16 255.255.255.240 28

32 255.255.255.224 27

64 255.255.255.192 26

128 255.255.255.128 25

256 255.255.255.0 24

See appendix C, Lists of Reserved Ports, ICMP Types and Codes, and Internet Protocols,

for more information on netmasks.

Name queries in 3Com VCX IP

Telecommuting Module

A Telecommuting Module should be as independent of other computers as possible. At the

same time, the person who changes the conﬁguration of the Telecommuting Module may

want to use names for the computers instead of IP addresses. Also, the SIP module needs to

look up names of SIP domains. This makes it necessary to use a DNS (name server) for SIP

requests.

There are three instances when 3Com VCX IP Telecommuting Module uses a DNS server:

• When it receives a SIP request for a SIP domain.

The results of these DNS queries are stored for a short while in the Telecommuting Mod-

ule.

• When you change names/IP addresses and save the page.