Introduction to working with IP Addresses

Probably work with decimal numbers much easier than with the binary numbers needed by the computer. Working with binary numbers is time-consuming & error-prone.

The 32-bit IP address is broken up into 4 octets, which are arranged into a dotted-decimal notation scheme. An octet is a set of 8 bits.

The binary system uses only 2 values “0 & 1” to represent numbers in positions representing increasing powers of 2.We all are accustomed to thinking & working in the decimal system, which is based on the number 10.

What then do you suppose is the largest decimal number that can be expressed in an octet?

11111111 (Eight 1’s)

Ø Converting to decimal

The largest decimal number that can be stored in an IP address octet is 255.

Ø IP Address Classes

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IP addresses are divided into 5 classes, each of which is designated with the alphabetic letters A to E. Class D addresses are used for multicasting. Class E addresses are reserved for testing & some mysterious future use.

Using the ranges, you can determine the class of an address from its 1^st octet value. An address beginning with 120 is a Class A address, 155 is a Class B address & 220 is a Class C address.

Each Network is assigned a network address & every device or interface (such as a router port) on the network is assigned a host address. There are only 2 specific rules that govern the value of the address.

A host address cannot be designated by all zeros or all ones. These are special addresses that are reserved for special purposes.

Ø Class A Addresses

v Class A IP addresses use the 1st8 bits (1^st Octet) to designate the Network ID.

v The 1stbit which is always a 0, is used to indicate the address as a Class A address & the remaining 7 bits are used to designate the Network.

v The other 3 octets contain the Host ID.

v There are 128 Class A Network Addresses, but because addresses with all zeros aren’t used & address 127 is a special purpose address, 126 Class A Networks are available.

v There are 16,777,214 Host addresses available in a Class A address.

v  The following formula to compute the number of hosts available in any of the class addresses, where “n” represents the number of bits in the host portion.

 

Half of all IP addresses are Class A addresses. You can use the same formula to determine the number of Networks in an address class.

Class A address uses 7 bits to designate the network, so (27– 2) = 126 or there can be 126Class A Networks.

Ø Class B IP Addresses

v Class B addresses use the 1st16 bits (two octets) for the Network ID.

v The last 2 octets are used for the Host ID.

v The 1st2 bit, which are always 10, designate the address as a Class B address & 14 bits are used to designate the Network. This leaves 16 bits (two octets) to designate the Hosts.

v There can be 16,382 Class B Networks & each Network can have 65,534Hosts.

Ø Class C IP Addresses

v Class C addresses use the 1^st 24 bits (three octets) for the Network ID & only the last octet for Host ID.

v The 1^st 3 bits of all class C addresses are set to 110, leaving 21 bits for the Network ID, which means there can be 2,097,150Class C Networks, but only 254 Hosts per Network.

Special Addresses

v A few addresses are set aside for specific purposes.

v Network addresses that are all binary zeros, all binary ones & Network addresses beginning with 127 are special Network addresses.