Lan Protocols

Lan Protocols:

The following sections will introduce the core LAN protocols that you will need to know for the exam.

TCP/IP:

Every IP address can be broken down into 2 parts, the Network ID(netid) and the Host ID(hostid). All hosts on the same network must have the same netid. Each of these hosts must have a hostid that is unique in relation to the netid. IP addresses are divided into 4 octets with each having a maximum value of 255. We view IP addresses in decimal notation such as 124.35.62.181, but it is actually utilized as binary data so one must be able to convert addresses back and forth. 

The following table explains how to convert binary into decimal and visa versa: 

Decimal	Binary	Explanation
128	10000000	When converting binary data to decimal, a "0" is equal to 0. "1" is equal to the number that corresponds to the field it is in. For example, the number 213 would be 11010101 in binary notation. This is calculated as follows: 128+64+0+16+0+4+0+1=213. Remember that this only represents 1 octet of 8 bits, while a full IP address is 32 bits made up of 4 octets. This being true, the IP address 213.128.68.130 would look like 11010101 10000000 01000100 10000010.
64	01000000
32	00100000
16	00010000
8	00001000
4	00000100
2	00000010
1	00000001

IP addresses are divided into 3 classes as shown below: 

Class	Range	Explanation
A	1-126	IP addresses can be class A, B or C. Class A addresses are for networks with a large number of hosts. The first octet is the netid and the 3 remaining octets are the hostid. Class B addresses are used in medium to large networks with the first 2 octets making up the netid and the remaining 2 are the hostid. A class C is for smaller networks with the first 3 octets making up the netid and the last octet comprising the hostid. The later two classes aren’t used for networks.
B	128-191
C	192-223
D	224-239 (Multicasting)
E	240-255 (Experimental)

A subnet mask blocks out a portion of an IP address and is used to differentiate between the hostid and netid. The default subnet masks are as follows: 

Class	Default Subnet	# of Subnets	# of Hosts Per Subnet
Class A	255.0.0.0	126	16,777,214
Class B	255.255.0.0	16,384	65,534
Class C	255.255.255.0	2,097,152	254

In these cases, the part of the IP address blocked out by 255 is the Net ID. 

In the table above, the it shows the default subnet masks. What subnet mask do you use when you want more that 1 subnet? Lets say, for example, that you want 8 subnets and will be using a class C address. The first thing you want to do is convert the number of subnets into binary, so our example would be 00001000. Moving from left to right, drop all zeros until you get to the first "1". For us that would leave 1000. It takes 4 bits to make 8 in binary so we add a "1" to the first 4 high order bits of the 4th octet of the subnet mask(since it is class C) as follows: 11111111.11111111.11111111.11110000 = 255.255.255.240. There is our subnet mask. 

Lets try another one...Lets say that you own a chain of stores that sell spatulas in New York and you have stores in 20 different neighborhoods and you want to have a separate subnet on your network for each neighborhood. It will be a class B network. First, we convert 20 to binary - 00010100. We drop all zeros before the first "1" and that leaves 10100. It takes 5 bits to make 20 in binary so we add a "1" to the first 5 high order bits which gives: 11111111.11111111.11111000.00000000 = 255.255.248.0. The following table shows a comparison between the different subnet masks. 

Mask	# of Subnets	Class A Hosts	Class B Hosts	Class C Hosts
192	2	4,194,302	16,382	62
224	6	2,097,150	8,190	30
240	14	1,048,574	4,094	14
248	30	524,286	2,046	6
252	62	262,142	1,022	2
254	126	131,070	510	Invalid
255	254	65,534	254	Invalid

Note: 127.x.x.x is reserved for loopback testing on the local system and is not used on live systems. 

TCP/IP Ports - Ports are what an application uses when communicating between a client and server computer. Some common TCP/IP ports are:

20 FTP-DATA

21 FTP

23 TELNET

25 SMTP

69 TFTP

70 GOPHER

80 HTTP

110 POP3

137 NetBIOS name service

138 NetBIOS datagram service

139 NetBIOS

161 SNMP

You need to understand Buffering, Source quench messages and Windowing. Buffering allows devices to temporarily store bursts of excess data in memory. However, if data keep arriving at high speed, buffers can go overflow. In this case, we use source quench messages to request the sender to slow down. 

Windowing is for flow-control purpose. It requires the sending device to send a few packets to the destination device and wait for the acknowledgment. Once received, it sends the same amount of packets again. If there is a problem on the receiving end, obviously no acknowledgement will ever come back. The sending source will then retransmits at a slower speed. This is like trial and error, and it works. Note that the window size should never be set to 0 - a zero window size means to stop transmittion completely. 

3COM’s IP addressing tutorial is just superior. It covers basic IP addressing options as well as subnetting and VLSM/CIDR. 

IPX/SPX:

IPX will also be an important issue to consider in network management given the fact there many companies still use Netware servers. There are two parts to every IPX Network address - the Network ID and the Host ID. The first 8 hex digits represent the network ID, while the remaining hex digits represent the host ID, which is most likely the same as the MAC address, meaning we do not need to manually assign node addresses. Note that valid hexadecimal digits range from 0 through 9, and hexadecimal letters range from A through F. FFFFFFFF in hexadecimal notation = 4292967295 in decimal. 

Sequenced Packet Exchange(SPX) belongs to the Transport layer, and is connection-oriented. It creates virtual circuits between hosts, and that each host is given a connection ID in the SPX header for identifying the connection. Service Advertisement Protocol(SAP) is used by NetWare servers to advertise network services via broadcast at an interval of every 60 minutes by default.