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Network FAQs (Frequently Asked Questions)

I'll be adding more questions to the list below as time permits. For now, I'm just gonna put the answer to some questions that I frequently forget:

What ranges of IP addresses are reserved for use as private networks?
There are three different network ranges which are designed for use as private networks. Machines with IP addresses within these ranges can be reached only within connected LANs; they cannot be reached directly from the Internet without a layer of address translation. These ranges are as follows:

Beginning Number
Ending Number
Classless Notation
10.0.0.0
10.255.255.255
10.0.0.0/8
172.16.0.0
172.31.255.255
172.16.0.0/12
192.168.0.0
192.168.255.255
192.168.0.0/16

 

Cisco Routers

How can I tell how long my router has been up?
The "show version" command will display this... and tons of other info, too, such as well.

 

Web Servers

Why is my web server's error log filling up with failed requests for "favicon.ico"?
Modern browsers have a feature which allows for a custom icon in the bookmark list, or in the address bar. In the past, if you saw a request for "favicon.ico," that meant someone had bookmarked your site. Now, however, I believe many browsers automatically request the file and insert it at the edge of the address bar.

 

Cabling

What is the difference between Cat-5 cable, and Cat-5e?
Category 5 cable is sufficient for 100-Mbps transmission. Category 5e cable, also known as Enhanced Category 5, conforms to a slightly more strict standard that guarantees full-duplex 100-Mbps Ethernet. That, of course, raises the question: what is full-duplex ethernet? A full-duplex connection allows simultaneous two-way transmissions in both directions. On a more traditional, half-duplex UTP cable, only two pairs of wires are used (one pair for transmitting, one pair for receiving). In full-duplex operation, all four pairs of wires are used.

A related note: ever hear of Category 4 cable? Yes, there is such a thing, and if you're a network admin, I hope for your sake you never encounter it. The main difference between Cat-4 and Cat-5 is the number of twists in the pairs of wire. More twists means less noise, and therefore higher speeds. Once upon a time I worked in an office where we'd been running all of our traffic through a tangled mess of old 8-port Ethernet hubs. I bought a nice, new 24-port dual-speed 10/100 hub, and suddenly started getting random failures throughout the entire office. Things would be fine for two or three days, and then poof... nobody could connect to anything. Eventually I determined that the failures usually occured shortly after someone connected a laptop to the network. That was when the lightbulb went off: the laptops had 10/100 network cards, and the old desktops didn't. I climbed up into the ceiling, grabbed a loose strand of cable, and read in tiny letters on the side: LEVEL 4. Ohhhh... level 4 cable is designed to support speeds up to only 20-Mbps. Ouch. Rewiring the whole office was out of the question, so I did what any slob would do: I stuck a 10-Mbps hub between the patch panel and the new hub to slow down everything except the Cat-5 backbone in the server room. Oh, well... at least it looked better than the maze of dinky little 8-ports.

VPNs

Every time I connect to a VPN using the Cisco client, my computer gets horribly, horribly slow. What's wrong?

Depending on the configuration of the VPN, you may or may not be able to access resources on your local area network. If access to the LAN is blocked, you won't be able to connect to things on your own network. (There is a client setting to allow or disallow access to your LAN, but it can be overridden by settings on the VPN server, or by policies on the network you are connecting to.) In this case, it makes perfect sense that your computer will perform badly any time you try to access one of your own network shares or printers; until the request times out, your computer will be more or less unresponsive. But why would everything be slow? Try to open your C: drive, and it's slow... try to open Word, and it's slow... try to open the web browser, and it's slow, slow, slow.

If this happens, check your profile settings in Active Directory. If the home directory is on the network, you'll suffer poor performance while connected to the VPN. Why are tasks which shouldn't need to access the network affected by this? I don't know... but they are. One way to test if this is indeed what is causing your problems is to log on to your computer using a local account, rather than a domain account, or to temporarily change the profile in AD.