20.          1.     The term "Segments" is usually associated with Transport layer

2.     The term "Packets" is usually associated with Network Layer and

3.     The term "Frames" is usually associated with Data Link Layer

21. The port number used by TCP is 6 and that of UDP is 17.

TCP: TCP is a full-duplex, connection-oriented protocol. It incorporates error checking as well.

UDP (User Datagram Protocol): UDP is a thin protocol. UDP is a connectionless protocol. It doesn't contact the destination before sending the packet and doesn't care whether the packet is reached at the destination. UDP uses port number 6.

22.  Switching methods:

1. Store-and-Forward switching: Here the LAN switch copies the entire frame into its buffers and computes the CRC. The frame is discarded if there are any CRC errors. Giant ( more than 1518 bytes0 and Runt (less than 64 bytes) frames are also dropped, if found.

   2. Cut-Through (Real-Time) switching: Here, the LAN switch copies only the destination address into its buffers. It immediately looks up the switching table and starts forwarding the frame. The latency is very less because, the frame is forwarded as soon as the destination address is resolved.

   3. Fragment-Free switching: Here, the switch waits for the collision window before forwarding the entire frame. The collision window is 64 bytes long.

23.  WAN (Wide Area Network) devices extend the reach of  LAN (Local Area Network) devices. WANs typically span over a wide area, such over multiple cities / countries. WANs are connected over serial lines that operate at lower speeds than LANs. Some of the WAN devices are:

1.     Routers: Routers are responsible for routing the packets in an internetwork.

2.     Modems: Modems connect to public telephone circuits through dial-up.

3.     CSU/DSU: Stands for Channel Service Unit / Data Service Unit. CSU/DSUs are used for connecting to Central Office of a Telephone switching company and provides serial WAN connections.

4.     Communication Servers: These are used for dial in/out to remote users. Provides RAS Remote Access Server) functionality.

5.     Multiplexers (mux): Multiplexers combine two or more signals before transmitting on a single channel. Multiplexing can be done by sharing "time" or "frequency".

24. Standard adopted for Ethernet CSMA/CD by IEEE Committee is 802.3. 100BaseT (Fast Ethernet) uses IEEE803.2u standard which incorporates CSMA/CD protocol.

25. DOD Model maps to OSI model as below:                   

26.  Routers can make alternate route decisions based on ICMP messages, if appropriate. Routers send an ICMP message if the destination is unreachable.

27. Port numbers are used by TCP/ UDP to set up sessions with other hosts. The following are the recommended port numbers:

1.     Numbers 0 - 255 are used for public applications.

2.     Numbers 255 - 1023 are assigned to companies so that they can use these port numbers in their applications.

3.     Numbers above 1023 are used by upper layers to set up sessions with other hosts and by TCP to use as source and destination addresses.

28. Ethernet_II has a type field to identify the upper-layer protocol. 802.3 has only a length field and can't identify the upper-layer protocol.

29.To find valid hosts given an IP address (say 156.16.3.52) and a subnet mask (sat a 12-bit subnet). The valid hosts are determined as below:

A 12-bit subnet mask gives us 255.255.255.240; 4094 subnets, each with 14 hosts. (Host addresses of all zeros and all 1s can't be assigned). The 156.16.3.52 is in the 48 subnet range. The valid range is through 49 - 62. 63 is a broadcast address.

Here is how you get the subnet range:

1.      Find the subnet mask. In this case, default subnet mask for Class B address is 255.255.0.0. There are additional; 12 bits in the subnet mask. Now the subnet mask looks like:

11111111.11111111.11111111.11110000. This is equal to 255.255.255.240.0.

2.      Now, deduct the lowest value octet (Which is non zero), from 256. Here, (256-240)=16.

This is the value that the subnets are incremented. Therefore, you will have hosts with values from:

156.16.3.1   to 156.16.3.14 (All 0s and all 1s host addresses can not be used)

156.16.3.17 to 156.16.3.30

156.16.3.33 to 156.16.3.46

156.16.3.49 to 156.16.3.62 and so on.

It is important to know that subnets are incrementing here by a factor of 16.

30. The following are some important commands that can be used to edit and review command history buffer on a Cisco router. It will be useful to practice these commands.

<ctrl> A : Move to the beginning of the command line

<ctrl> E : Move to the end of the command line

<ctrl> F : Move forward one character, same as using "Right Arrow".

<ctrl> B : Move backward one character, same as using "Left Arrow".

<ctrl> P : Repeat Previous command, same as using "Up Arrow".

<ctrl> N : Repeat Next (more recent) command, same as using "Down Arrow".

<esc> B : Moves to beginning of previous word.

<esc> F : Moves to beginning of next word.

<ctrl>R : Creates new command prompt, followed by all the characters typed at the last one.

Any comments or feedback on the above notes may please be directed to the contributor here