That All Depends...
It all depends on what you're actually making for supporting that many systems/clients. My network isn't quite as large, but I'll bet I'm only making 3/4 of your salary.;) Has the network grown a lot since you last had a raise? That's usually a good leverage point. The company WANTS you to think of raises as 'taboo' because that's money in their pocket and not yours.
What I would do is invite your manager/boss/etc. out to lunch sometime, or just schedule a meeting with them. Outline how your duties have grown over the past x years and that, although you love your job, the lack of a monetary increase has been causing you to love it a little less. They'll try to steer the conversation towards something else (probably how the company is struggling to stay afloat) but note how your salary compares to the average, and how the duties you perform are more than worth it. Stick to your guns. If they don't offer you something by the end of the meeting, mention casually that you still enjoy working there, but you'll be on the lookout for other opportunities that might arise.
That's about all you can do without threatening to leave out-right. But remember, our pay is considered small potatoes against the total revenue and operating budget of the company. I've overheard execs talking on the golf course about how cheaply they're getting away with their IT support because 'their guy is happy with ONLY making that much.'
