If you are having lag in game, seeing yellow, red or excessive spikes in the Lagometer, experiencing long times in awaiting gamestate, or seeing AntiWarp triggered a lot in etpro, you can do some basic testing on your connection with ping and tracert. The following examples use windows command names and options, linux is slightly different.
First, tracert to somewhere (don't really matter, use the IP of an ET server you like to play on, or your ISPs website or google) like this, in a command prompt:
The output will be something like this:
Tracing route to www.l.google.com [22.214.171.124] over a maximum of 30 hops: 1 <1 ms <1 ms <1 ms 192.168.1.1 [192.168.1.1] 2 14 ms 13 ms 13 ms rdns.of.your.isp.gateway [126.96.36.199] 3 13 ms 14 ms 13 ms rdns.of.some.other.router[188.8.131.52] ...
The first hop (with the < 1ms ping times) either your router or modem. The next hop is usually the far end of your broadband line. If thing ping time of the second hop is also very low (<10ms) it is your modem, and the hop after that is the other end. Don't be alarmed if some of the later hops drop packets in tracert, as a fair number of routers drop these, even when they forward normal traffic without problems. In general, if one hop drops all packets, but the subsequent hops still work, it just means that particular device doesn't respond to tracert packets.
To test your connection between you and your router, use:
ping -n 100 -l 1000 <ip.of.your.router>
The -n 100 means to use 100 packets, and the -l 1000 means use 1000 byte packets (that's a lower case L). Larger packets are far more likely to expose line problems.
This should give zero packet loss and <10ms pings. If it gives you packet loss, either your router sucks, your Ethernet cable is bad, or perhaps some setting for your Ethernet card is incompatible.
To test your line (the connection from your house to your ISPs equipment), find the IP of the first hop that isn't your router or modem (i.e. the first one that has a >10ms time in the tracert output), and ping it with the same command. Any packet loss most likely indicates a problem (unless its 100%, in which case, it is likely that router is simply ignoring your pings). The pings times should be fairly steady if there is no other traffic on the line, typically in the range of 20-80 ms for DSL or cable with this packet size. If there is any significant packet loss, chances are you need to complain to your ISP. If there is significant variation (+/- 20% or more) and your line does not have other traffic on it, that may also indicate a problem.
You can move on down the IPs listed in the tracert using the same method, if you want, however, if the problem is in a router that is more than a couple hops away, chances are it is a temporary issue, and you can't do much to get it fixed.