how should i choose a port number for my server
I will answer some of your questions, you can answer the other through WiKi, it will help you better than me.
* Layers in TCP/IP
4 layers: Application + Transport + Internet + host-to-network
* How can I be sure that a UDP message is received?
I think this is a trick question: you can't make sure that UDP packet arrived because it didn't transmit any acknowledgment about reciving transmitting packet like TCP, you should go to the other end to make sure your packet recieved.
* Can I connect two computers to internet using same line ?
Yes, you can connect the two computers through Cross-Cable, connect on of them to the internet and made the modem of that Computer shared though ICS (Internet Connection Sharing).
* Why networks are layered ? What is the advantage of that ?
Read about IOS Model
* User(s) are complaining of delays when using the network. What would you do?
Check their connectivity between users and your resources, maybe there is HUB causing high collision or you are using only 10Mb port speed or there is a high broadcast traffic in your network because you are using only HUB and Switches instead or router or VLAN.
* What are some of the problems associated with operating a switched LAN?
Read about VLAN
Broudcast traffic will be heard among all users and you can't manage or secure your LAN's like VLAN.
* What sort of cabling is suitable for Fast Ethernet protocols?
Cat5 Cat6 Cat7
* Why do I sometimes lose a server's address when using more than one server?
Your question is not clear.
Do you mean "when using more than one server" using more than one DNS server, in that case maybe the entry between the DNS Servers were not updated yet, in that case you may lose your server's address.
* How many layers are there in OSI ? Why is it called OSI model ?
7 layers
Regards,