(This is an attempt to explain, to a non-technical audience, why a large network, such as the Brandeis network, should be divided into subnets. It comes from this bboard thread.)
The reasons why one wouldn’t want to have the entire campus be one huge subnet are technical, but I’ll try to explain them. Think of the Brandeis network as a huge party. It’s a tremendous party, with every student, faculty-member, and staff-member in one huge room. The university is sponsoring it to celebrate the grand opening of the Carl and Ruth Shapiro Supremely Massive Empty Room.
You think your friend Alice is at the party, and you want to talk to her. So, you have to find her. You don’t know where she is in the massive room, so you have to either shout really loudly, or spend a lot of time looking for her. Everyone else is also trying to find people, so everybody’s shouting and running around.
Furthermore, people are trying to get to different places in the room. Once people have found their friends, they need to walk over to them, plus people need to get to the exits, the bathrooms, the bar, the DJ, and so forth. Some people are even trying to dance. So, everybody is walking everywhere, and dancing, and shouting. This might be fun, if you’re into that kind of thing, but it’d be really hard to find Alice, walk over to her without bumping into people and without taking some crazy zig-zagging route, and have a conversation with her.
Then it gets worse. Bob, that insufferable boor, has too much to drink, and gets in a fight with Charlie. Their shouting and scuffling drown out the rest of the activity in the room, and makes it impossible for anyone to have a good time until Public Safety comes and escorts them out. It takes a long time for Public Safety to locate Bob and Charlie in the room, despite their loudness, because the crowd is so thick, and even when Bob and Charlie are found, it takes many minutes for the officers to push through the crowd and get the drunks removed.
Well, the party is a disaster. Carl and Ruth get separated by people running between them trying to get to Alice, and they can’t find each other until 3AM. Jehuda spends the whole night trying to dance the Electric Slide, but people are shouting over the music and he can’t keep the beat.
To make things up to the Shapiros, Brandeis decides to throw another party. This time, they decide to get Brandeis’s most renowned social event planners to help them out with it, and so they go to the ITS department, whose infamous VoIP Rollout Bash is still whispered about in revered tones.
ITS throws the party in a large house, with many different rooms. Each room has between fifty and a couple of hundred people in it. Furthermore, people are assigned to rooms alphabetically, so you know which room everyone is in. Each room has a robot in it which can relay sounds, 3D images, smells, and even the sensation of someone dancing with you, from one room to another. This robot is so powerful, that all you have to do is say, at a normal conversational volume, “Hi, Alice?”, and the robot will zip into the corridor, fly down to some out-of-the-way closet, and give the message to a page, which will zip it over to the robot in Alice’s room. The robot is almost never too busy, the layout of the corridors is so efficient that messages can get between rooms instantaneously.
People can find each other and talk to each other, and are having a good time. You and Alice dance the night away. Bob and Charlie get drunk and start fighting again, but only the people in their room are affected, since the robot doesn’t pass on their alchohol-fueled shouting; also, Public Safety knows what room they’re in, and because the room is much smaller they can get them escorted out much more quickly. The party is a smashing success. Jehuda wins the dance competition, and Carl and Ruth have a wonderful time.