When I think I must have a really stupid question, I remind myself that there most likely a number of others that have the same question they would like to ask, but are reluctant to do so. So here goes:
Just how does one go about "pinging," a computer?
Depends on the OS. From Mac OS X's terminal window, you can just use a ping command. From the classic Mac OS, you have to use a utility that supports such.
My two fav pingers are IPNetMonitor (http://www.sustworks.com/) and WhatRoute (http://www.whatroute.net/). The former is shareware, the latter is freeware and was included on later Mac OS CDs.
And what information does one receive from "pinging" and what does one do with it to accomplish what?
PING is a reference to the use of sonar on submarines. You send out a PING then wait for the echoing PONG. By receiving the pong, you can tell that the target is out there. By timing it, you can tell (more or less) how far away it be.
In computers, we use ping to test basic ip connectivity. You send an ICMP Echo Request (ping) and the target hopefully responds with an Echo Response (pong).
eg:
<ping:www.yahoo.com>
0.030 216.109.118.71 p8.www.dcn.yahoo.comSo Yahoo is responding at the ip address "216.109.118.71" and the round trip (ping+pong) lag (or latency) is 30ms -- it is 15ms away.
Now... if you combine pings and give them variable time-to-lives, you can move along the path through the internet that your packets use, testing each router along the say. This is called a traceroute.
eg: (using IPNetMonitor)
<traceroute:www.yahoo.com>
Hop Sent Received Seconds IP Address Name
1 YYY YYY 0.005 192.168.0.1 No host name found.
2 YYY YYY 0.021 10.126.172.1 No host name found.
3 YYY YYY 0.016 68.87.50.49 fe-2-6-rr01.cherryhillea.nj.nj04.comcast.net
4 YYY YYY 0.016 68.87.50.161 No host name found.
5 YYY YYY 0.018 68.87.19.209 pos-7-2-cr01.torresdale.pa.core.comcast.net
6 YYY YYY 0.018 12.119.53.49 No host name found.
7 YYY YYY 0.023 12.123.137.45 tbr1-p012401.phlpa.ip.att.net
8 YYY YYY 0.024 12.122.2.17 tbr1-cl8.n54ny.ip.att.net
9 YYY YYY 0.023 12.123.3.58 ggr2-p300.n54ny.ip.att.net
10 YYY YYY 0.028 192.205.32.198 att-gw.ny.cw.net
11 YYY YYY 0.034 206.24.226.99 dcr1-loopback.washington.savvis.net
12 YYY YYY 0.029 206.24.238.38 bhr1-pos-10-0.sterling2dc3.savvis.net
13 YYY YYY 0.029 216.109.84.162 216.109.84.162
14 YYY YYY 0.029 216.109.120.142 vl30.bas1-m.dcn.yahoo.com
15 YYY YYY 0.029 216.109.118.66 p3.www.dcn.yahoo.com
Each router is pinged three times and responded three times (the Y's).
My Mac is on my private LAN, at 192.168.0.1.
My NAT Router is being shy, so it's not there (hop 1.5)
Hop 2 is my cable company's CMTS (head end router). This is the so-called first/last mile hop. 21ms is decent. If it was 50ms+, I'd be worried that something was wrong with my modem, coax, or their CMTS...
Now, if a circuit or router was having problems, you'd see either the ping failing or the average time increase. By comparing tracroutes and extra pings, you can often figure out what's broken where.
HTH, - Dan.
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