We've surveyed the workings of a single computer system — the physical machine, OS and programs. We've also looked at how the World Wide Web, a system that's comprised of millions of web servers and billions of browsers communicating with one another. The web is just one example of a system built on top of computer networks — the Internet, in particular. We will now turn our attention to how the Internet works and, later, to how wired and wireless networks work in general. This class will be our first step.
220.127.116.11, though more on that later.
18.104.22.168 ___/ / \ \____ / / \ \ 10010010.10010001.00000101.01000011 → 10010010100100100000010101000011 → 2458977603The reason we prefer dotted quad notation is a) they're generally easier to remember, and b) the first two or three bytes are often the same for all IP Addresses within a given organization. For example, IP Addresses in Michelson Hall often begin with 10.53.*.* or 10.22.*.*. The similarity is harder to see when addresses are written out as a single 32-bit number.
ipconfigin the shell. (1. Open command prompt. 2. Type
ipconfig) There will be a line in the output that looks like
IPv4 Address. . . . . . : 10.22.88.124and that, of course, is your IP Address. It'll be really important throughout the course to be able to answer when someone says "what's your IP Address?".
At this 2008 conference sponsored by Google, Vinton Cerf tells us why. Listen to the discussion at 13:00 - 14:35. Vinton Cerf, Project Director for the TCP/IP research program at DARPA in 1976, is at the podium. Bob Hinden, who is seated, helped develop the first Internet routers. [Embedded by permission of the YouTube Terms of Service]
ipconfigto see if you can find both.
www.usna.edu. This kind of name is called a domain name.
/edu/usna. In a domain name, the more specific portion is to the left, the more general portion is to the right. For example,
intranet.usna.eduis a subset of
usna.edu, which is in turn a subset of
.edu, which is called a "top-level domain".
wwwat the front of a name, like
www.usna.eduusually is meant to indicate a web server, but having
wwwat the front of a URL doesn't make a machine a web server any more than having the first name "Prince" makes you royalty. It's just a convention.
ipconfig /all. Look for the line
DNS Servers. . . . . : 10.1.74.10that contains the IP Addresses of one or more name servers.
nslookuputility is a shell tool (for both Windows and UNIX) that will carry out a DNS request for you. Here's an example:
$ nslookup yog.academy.usna.edu Server: ns1.usna.edu Address: 10.1.74.10 Name: yog.academy.usna.edu Address: 10.1.83.30From this we see that the IP Address of the host
10.1.83.30. Furthermore, the output is telling us that the name server that provided us this answer has IP Address
nslookuputility is also able to do reverse DNS requests — i.e. "here's an IP Address, what's the name?". We can use that to find the name of the name server we just queried.
$ nslookup 10.1.74.10 Server: ns1.usna.edu Address: 10.1.74.10From this we see that the name server 10.1.74.10 has the name
nslookupwill query the name server listed by the call to
ipconfig /allto do DNS lookups. However, if you call
nslookupwith a second argument that is the name or IP Address of a name server,
nslookupwill query that name server instead. So, for example:
$ nslookup www.google.com 22.214.171.124... actually causes my host to contact
126.96.36.199to resolve the name
www.google.com. However, if I run this request inside the USNA network, it can't complete, because (for security reasons), USNA does not want DNS requests to be fulfilled by outside (potentially untrusted) name servers. The firewall blocks the request.
http://www.martinguitar.com/in your browser's address bar, the browser is supposed to send a request to the web server www.martinguitar.com (specifically an HTTP GET request). But that can't happen until the browser finds out what IP Address goes with that name. In fact, you can enter the IP Address directly into the browser's address bar, like this
http://188.8.131.52... and you'll get the website (Side note: this trick occasionally can run into problems if the particular web server or browser requires use of the HTTP/1.1 protocol rather than HTTP/1.0, but in general a web site is accessible via its IP Address only, if you know the address). If you use the symbolic name, however, the browser first makes a DNS request to a name server to get the IP Address for the name www.martinguitars.com, and then actually sends the HTTP GET request to the web server. If you don't have access to a name server, and you know only a web site's URL, not its IP Address, you can't access the web site!
"Nippernet" is the colloquial name for the DoD subset of the Internet that carries "sensitive but UNCLASSIFIED IP data". Access to the NIPRNet is tightly controlled: all data crossing the NIPRNet/Internet boundary must pass through a DoD-owned router, and hosts on the NIPRNet resolve names using DNS servers operated by the DoD Network Information Center. (The DoD also owns and operates the SIPRNet, for CLASSIFIED data.)
traceroutethat shows the path a single packet travels from a source host to a destination host on the Internet. The following output from
tracerouteshows the route from a host here at USNA to a host (in this case, a web server) in Austria.
tracert. Try it from your laptop.
$ traceroute www.risc.uni-linz.ac.at 1 michelson-3a-as1-v401.gw.usna.edu (10.53.33.1) 1.779 ms 1.180 ms 1.167 ms 2 michelson-1a-ag1-v2x05.net.usna.edu (10.48.1.93) 1.200 ms 1.274 ms 0.904 ms 3 yard-d2-v3x60.net.usna.edu (10.48.2.81) 0.776 ms 0.681 ms 0.734 ms 4 usna-c2-v2x14.net.usna.edu (10.0.2.25) 0.922 ms 0.786 ms 0.805 ms 5 border-d2-v2x18.net.usna.edu (10.0.2.34) 1.533 ms 0.992 ms 1.011 ms 6 border-f2-gi0_8s3001.net.usna.edu (10.4.16.2) 1.506 ms 1.044 ms 1.230 ms 7 maryland-r1-po1s2001.net.usna.edu (184.108.40.206) 1.335 ms 1.714 ms 1.487 ms 8 gig1-8.umab-core.net.ums.edu (220.127.116.11) 2.477 ms 2.763 ms 12.486 ms 9 balt-usm.maxgigapop.net (18.104.22.168) 4.015 ms 3.218 ms 3.271 ms 10 i2-newy-rtr.maxgigapop.net (22.214.171.124) 11.083 ms 11.311 ms 12.932 ms 11 126.96.36.199 (188.8.131.52) 110.292 ms 110.289 ms 109.886 ms 12 ae3.mx1.ams.nl.geant.net (184.108.40.206) 97.495 ms 110.193 ms 97.360 ms 13 ae0.mx1.fra.de.geant.net (220.127.116.11) 91.859 ms 95.822 ms 104.412 ms 14 ae0.mx1.pra.cz.geant.net (18.104.22.168) 98.401 ms 98.421 ms 101.505 ms 15 ae2.mx2.bra.sk.geant.net (22.214.171.124) 103.908 ms 125.384 ms 114.995 ms 16 ae0.mx1.vie.at.geant.net (126.96.36.199) 116.916 ms 130.063 ms 117.406 ms 17 aconet-gw.rt1.vie.at.geant.net (188.8.131.52) 117.472 ms 119.264 ms 129.666 ms 18 vlan73.wien21.aco.net (184.108.40.206) 129.450 ms 116.790 ms 116.867 ms 19 vlan71.wien1.aco.net (220.127.116.11) 116.849 ms 135.726 ms 117.317 ms 20 vlan311.linz1.aco.net (18.104.22.168) 132.574 ms 119.865 ms 132.131 ms 21 jku-gw.edvz.uni-linz.ac.at (22.214.171.124) 132.455 ms 120.046 ms 120.938 ms 22 jkuc3hb1.edvz.uni-linz.ac.at (126.96.36.199) 120.389 ms 120.000 ms 120.348 ms 23 router.risc.uni-linz.ac.at (188.8.131.52) 120.341 ms 120.172 ms 120.265 ms 24 crow.risc.uni-linz.ac.at (184.108.40.206) 132.354 ms 132.261 ms 120.308 ms