IP Addresses, Classes, Subnets, CIDR, DNS - IPv4 and IPv6

The Internet Protocol - IP - was created in the 1970s to support early computer networking with the Unix operating system. Today, IP has become a standard for all modern network operating systems (NOS) to communicate with each other. Many popular higher-level protocols such as HTTP and TCP rely on IP.
Two versions of IP exist in production use today. Nearly all networks use IP version 4 (IPv4), but an increasing number of educational and research networks have adopted the next generation IP version 6 (IPv6).

IPv4 Addressing Notation
An IPv4 address consists of four bytes (32 bits). These bytes are also known as octets.
For readability purposes, humans typically work with IP addresses in a decimal notation that uses periods to separate each octet. For example, the IP address

00001010 00000000 00000000 00000001

usually appears in the equivalent dotted decimal representation

10.0.0.1

Because each byte is 8 bits in length, each octet in an IP address ranges in value from a minimum of 0 to a maximum of 255. Therefore, the full range of IP addresses is from 0.0.0.0 through 255.255.255.255. That represents a total of 4,294,967,296 possible IP addreses.

IPv6 Addressing Notation
IP addressing changes significantly with IPv6. IPv6 addresses are 16 bytes (128 bits) long rather than four bytes (32 bits). That represents more than

300,000,000,000,000,000,000,000,000,000,000,000,000

possible addresses! In the coming years, as an increasing number of cell phones, PDAs, and other network appliances expand their networking capability, this much larger IPv6 address space will probably be necessary.

IPv6 addresses are generally written in the following form:
hhhh:hhhh:hhhh:hhhh:hhhh:hhhh:hhhh:hhhh

In this notation, pairs of IPv6 bytes are separated by a colon and each byte in turns is represented as an equivalent pair of hexadecimal numbers, like in the following example:

E3D7:0000:0000:0000:51F4:9BC8:C0A8:6420

IPv6 addresses often contain many bytes with a zero value. Shorthand notation in IPv6 removes these values from the text representation (though the bytes are still present in the actual network address) as follows:

E3D7::51F4:9BC8:C0A8:6420

Finally, many IPv6 addresses are extensions of IPv4 addresses. In these cases, the rightmost four bytes of an IPv6 address (the rightmost two byte pairs) may be rewritten in the IPv4 notation. Converting the above example to mixed notation yields

E3D7::51F4:9BC8:192.168.100.32

IPv4 Address Classes

The IPv4 address space can be subdivided into 5 classes - Class A, B, C, D and E. Each class consists of a contiguous subset of the overall IPv4 address range.
With a few special exceptions explained further below, the values of the leftmost four bits of an IPv4 address determine its class as follows:


All Class C addresses, for example, have the leftmost three bits set to '110', but each of the remaining 29 bits may be set to either '0' or '1' independently (as represented by an x in these bit positions):

110xxxxx xxxxxxxx xxxxxxxx xxxxxxxx

Converting the above to dotted decimal notation, it follows that all Class C addresses fall in the range from 192.0.0.0 through 223.255.255.255.

IP Address Class E and Limited Broadcast

The IPv4 networking standard defines Class E addresses as reserved, meaning that they should not be used on IP networks. Some research organizations use Class E addresses for experimental purposes. However, nodes that try to use these addresses on the Internet will be unable to communicate properly.
A special type of IP address is the limited broadcast address 255.255.255.255. A broadcast involves delivering a message from one sender to many recipients. Senders direct an IP broadcast to 255.255.255.255 to indicate all other nodes on the local network (LAN) should pick up that message. This broadcast is 'limited' in that it does not reach every node on the Internet, only nodes on the LAN.
Technically, IP reserves the entire range of addresses from 255.0.0.0 through 255.255.255.255 for broadcast, and this range should not be considered part of the normal Class E range.

IP Address Class D and Multicast

The IPv4 networking standard defines Class D addresses as reserved for multicast. Multicast is a mechanism for defining groups of nodes and sending IP messages to that group rather than to every node on the LAN (broadcast) or just one other node (unicast).
Multicast is mainly used on research networks. As with Class E, Class D addresses should not be used by ordinary nodes on the Internet.

IP Address Class A, Class B, and Class C

Class A, Class B, and Class C are the three classes of addresses used on IP networks in common practice.

IP Loopback Address

127.0.0.1 is the loopback address in IP. Loopback is a test mechanism of network adapters. Messages sent to 127.0.0.1 do not get delivered to the network. Instead, the adapter intercepts all loopback messages and returns them to the sending application. IP applications often use this feature to test the behavior of their network interface.
As with broadcast, IP officially reserves the entire range from 127.0.0.0 through 127.255.255.255 for loopback purposes. Nodes should not use this range on the Internet, and it should not be considered part of the normal Class A range.
Zero Addresses

As with the loopback range, the address range from 0.0.0.0 through 0.255.255.255 should not be considered part of the normal Class A range. 0.x.x.x addresses serve no particular function in IP, but nodes attempting to use them will be unable to communicate properly on the Internet.

Private Addresses

The IP standard defines specific address ranges within Class A, Class B, and Class C reserved for use by private networks (intranets). The table below lists these reserved ranges of the IP address space.

Nodes are effectively free to use addresses in the private ranges if they are not connected to the Internet, or if they reside behind firewalls or other gateways that use Network Address Translation (NAT).

IPv6 Address Types

IPv6 does not use classes. IPv6 supports the following three IP address types:
> unicast
> multicast
> anycast

Unicast and multicast messaging in IPv6 are conceptually the same as in IPv4. IPv6 does not support broadcast, but its multicast mechanism accomplishes essentially the same effect. Multicast addresses in IPv6 start with 'FF' (255) just like IPv4 addresses.
Anycast in IPv6 is a variation on multicast. Whereas multicast delivers messages to all nodes in the multicast group, anycast delivers messages to any one node in the multicast group. Anycast is an advanced networking concept designed to support the failover and load balancing needs of applications.

IPv6 Reserved Addresses

IPv6 reserves just two special addresses: 0:0:0:0:0:0:0:0 and 0:0:0:0:0:0:0:1. IPv6 uses 0:0:0:0:0:0:0:0 internal to the protocol implementation, so nodes cannot use it for their own communication purposes. IPv6 uses 0:0:0:0:0:0:0:1 as its loopback address, equivalent to 127.0.0.1 in IPv4.
IP is used by many higher level network protocols, principally TCP and UDP. Many Internet software applications including Web browsers, FTP clients, and email programs, rely on Internet Protocol.
OSI Model: Network layer (Layer 3)
Datagram Format: A base header 20 bytes (5 "longwords") in length, with the option for expanded header options, followed by data
Header:

Word 1 -
Version - 4 bits
Header Length (in longwords) - 4 bits
Type of Service / Differentiated Services Code Point (DSCP) - 8 bits
Datagram Length (in bytes) - 16 bits

Word 2 -
ID Number - 16 bits
Fragmentation Flags - 3 bits
Fragmentation Offset - 13 bits

Word 3 -
Time to Live - 8 bits
Transport Protocol - 8 bits
Header Checksum - 16 bits

Word 4 -
Source IP Address - 32 bits

Word 5 -
Destination IP Address - 32 bits

Variable length fields -
Options
Padding

Payload: IP datagram payloads can be of variable length. The minimum size of an IP datagram is 28 bytes, using the minimum 20 bytes of header information, followed by the minimum of 8 bytes of data. The maximum size of an IP datagram payload is 65,535 bytes minus the header size.

Footer: Internet Protocol does not use its own datagram footer


- SRIKANTH DHANWADA
_________________
SD
Hardwaredude.com Team