a server is acting as a router between two networks. if the load increases what happens, to the speed of network? whether the network speed will be reduced or it will sustain the same speed(Ex: 100MBps)

That depends on the harddrive doesn't it? Or are you just routing through it without sharing files or data on that computer itself? Or clustering?

And what "load" exactly? The 100mbs network speed in full use or some app running or something else?

Normally it shouldn't have a problem, its only 12.5mb.

hi,

The server system consist of two network adapter, one network card have the ip address of 200.168.1.27 and other card is 200.168.10.27. The speed of network adapter is only 100MBps. 200.168.1.27 network serves as gateway to the first network and 200.168.10.27 network is a gateway of second network.
If number of systems included in both the networks increases, what happens to the speed(ofcourse, the speed will be shared among the system)?? Whether it makes the packets to reach its destination slowly?? Whether the speed shared among the systems will be effective??

If you wanna set the speed specifically than you should use a bandwidth sharing software, to set it exactly for example (like split 50/50 or make it so each machine can only use so much). Otherwise it will just share/split it and lets say you have two things sharing at the same time it could go 50/50 or more or less probably depending a bit on the machines behind it. Like running 2 simultaneous downloads.
If you run more machine it will divide it down.

The packets won't go slower, if full speed is in use it will lower the speed, divide/spilt it, in terms of mbits left (not in terms of "packets") when running more and more computers and sharing at the same time.

You should probably upgrade to gigabit net since its much much faster. And than if you do that also get a switch which can share preferably a gigabit of network speed at the same time, not all can so check that when you buy it. Based on full duplex high speed data processing.

Is it necessary to use router device between two network to have a control on the speed or Switch is sufficient?

Could you kindly tell me where i can get bandwidth sharing software

No you won't need a router, the right software will be sufficient.

I can't think currently which one exactly but if you search google.com I'm sure you'll find one.

Thank you so much for your valuable assistance.

No problem.