I'm new to this and haven't had much experience, so...

I friend gave me a 100ft. cable so I could move my router to the other end of my house. It's much bigger and doesn't work. I can connect it between my router and computer and access the router itself, but if I connect it between the modem and the router, I have no internet access.

The cable I was using (am using now, successfully) has this written on it:

BETTER (UL) E302151 CM 75c 4PR 24AWG C (UL) CM FT4 ETL VERIFIED TO TIA/EIA 568B.2 CAT.5 PATCH CORD---UTP


ok...so the new, 100ft'er has this written on it:

CABLES TO GO VERIFIED (UL) CAT5E E129760-C (UL) CMR 60c ETL VERIFIED TIA/EIA 568.2 CAT5E ENHANCED 350MHZ 4UTP 24AWG CSA LL80671 AWM II A/B/ 80c 30V FT1

Why won't it work? They both have the same RJ-45 connector on the end. And, how do I know what kind of cable to get when I go shopping for the 100ft'er that I really need, instead of this one?

Check the wiring of each connector end to see if it is right and to see if it straight through or crossover?

_________________
Networking is much like making love to a beautiful woman
Slide your equipment in to the rack, Stick your plug in the socket, and if you have done it right at the end everyone is happy

It's possible it's just a bad cable or it's terminated for 568A, and your using 568B. If you know anyone with a test see if they will test it for you.

Read this and take a look at the pins and see if you can which colors are in which positions.

http://en.wikipedia.org/wiki/TIA/EIA-568#T568A_and_T568B_termination

Ahhh cables eh??!!

Well, Ethernet Cat 5E is tested up to 100meters so 100ft should be fine.
You may have to check whether the cable is straight or crossover.
Hold the two RJ45 ends clip down and look for the internal cable colours.

if they both look like this:
orange white
orange
green white
blue
blue white
green
brown white
brown
...................it's straight.
if, however, one has the orange and green wires swapped on one side
it's crossed.
Do you get link activation lights on the devices? If not then it's the
wrong cable for the job or faulty. As I don't know the type of modem youh
ave I can't, with confidence, say whether a straight or crossover is
needed. If you have both then try both and see if you get link status lights.

I'm 99% sure it has nothing to do with the cable distance, but if it's crossover or not. You will sometimes get a link light but it won't connect if it's got the wrong pinout. i.e. crossover or not. silverbulletcouk is right.

568a or 568b are both fine, just one is usually residential, and one is commercial, but as long as they're the same on both sides it doesn't matter. The Orange and green pair just swap between 568a and 568b, but the numbering is the same, i.e. 12356478 where 1/2 are a pair, 3/4 are a pair, 5/6 is a pair, and 7/8 is a pair. http://pinouts.ru/ is my favorite site for these things.