Making Things Talk

A MORE COMPLEX NETWORK 87

To use it, open up the command-line application on your

computer (Terminal on Mac OS X, the Command Prompt

on Windows, and xterm or similar on Linux/Unix). On Mac

OS X or Linux, type the following:

ping -c 10 127.0.0.1

On Windows, type this:

ping -n 10 127.0.0.1

This sends a message to address 127.0.0.1 and waits for a

reply. Every time it gets a reply, it tells you how long it took,

like this:

64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.166 ms

64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.157 ms

64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.182 ms

After counting ten packets (that’s what the –c 10 on Mac

and -n 10 on Windows means), it stops and gives you a

summary like this:

--- 127.0.0.1 ping statistics ---

10 packets transmitted, 10 packets received, 0% packet loss

round-trip min/avg/max/stddev = 0.143/0.164/0.206/0.015 ms

It gives you a good picture not only of how many packets

got through, but also how long they took. It’s a useful

way to learn quickly if a given device on the Internet is

reachable or not, and how reliable the network is between

you and that device. Later on, you’ll be using devices that

have no physical interface that you can see activity on, so

you’ll need ping to know whether they’re working or not.

NOTE: 127.0.0.1 is a special address called the loopback address

or localhost address. Whenever you use it, the computer you’re ...