Internet Protocol: – Communication between hosts can happen only if they can identify each other on the network. In a single collision domain (where every packet sent on the segment by one host is heard by every other host) hosts can communicate directly via MAC address.MAC address is a factory coded 48-bits hardware address which can also uniquely identify a host. But if a host wants to communicate with a remote host, ie not in the same segment or logically not connected, then some means of addressing is required to identify the remote host uniquely. A logical address is given to all hosts connected to the Internet and this logical address is called Internet Protocol Address.
The network layer is responsible for carrying data from one host to another. It provides means to allocate logical addresses to hosts, and identify them uniquely using the same. Network layer takes data units from Transport Layer and cuts them in to smaller unit called Data Packet.
Network layer defines the data path, the packets should follow to reach the destination. Routers work on this layer and provides mechanism to route data to its destination. A majority of the internet uses a protocol suite called the Internet Protocol Suite also known as the TCP / IP protocol suite. This suite is a combination of protocols which encompasses a number of different protocols for different purpose and need. Because the two major protocols in this suites are TCP (Transmission Control Protocol) and IP (Internet Protocol), this is commonly termed as TCP / IP Protocol suite. This protocol suite has its own reference model which it follows over the internet. In contrast with the OSI model, this model of protocols contains less layers.
Internet Protocol Version 4 (IPv4)
Internet Protocol is one of the major protocols in the TCP / IP protocols suite. This protocol works at the network layer of the OSI model and at the Internet layer of the TCP / IP model. Thus this protocol has the responsibility of identifying hosts based upon their logical addresses and to route data among them over the underlying network.
IP provides a mechanism to uniquely identify hosts by an IP scheme. IP uses best effort delivery, ie it does not guarantee that packets would be delivered to the destined host, but it will do its best to reach the destination. Internet Protocol version 4 uses 32-bit logical address.
Internet Protocol being a layer-3 protocol (OSI) takes data Segments from layer-4 (Transport) and divides it into packets. IP packet encapsulates data unit received from above layer and add to its own header information.
The encapsulated data is referred to as IP Payload. IP header contains all the necessary information to deliver the packet at the other end.
IP header includes many relevant information including Version Number, which, in this context, is 4. Other details are as follows:
• Version: Version no. of Internet Protocol used (eg IPv4).
• IHL: Internet Header Length; Length of entire IP header.
• DSCP: Differentiated Services Code Point; this is Type of Service.
• ECN: Explicit Congestion Notification; It carries information about the congestion seen in the route.
• Total Length: Length of entire IP Packet (including IP header and IP Payload).
• Identification: If IP packet is fragmented during the transmission, all the fragments contain the same identification number. to identify original IP packet they belong to.
• Flags: As required by the network resources, if IP Packet is too large to handle, these 'flags' tells if they can be fragmented or not. In this 3-bit flag, the MSB is always set to '0'.
• Fragment Offset: This offset tells the exact position of the fragment in the original IP Packet.
• Time to Live: To avoid looping in the network, every packet is sent with some TTL value set, which tells the network how many routers (hops) this packet can cross. At each hop, its value is decremented by one and when the value reaches zero, the packet is discarded.
• Protocol: Tells the Network layer at the destination host, to which Protocol this packet belongs to, ie the next level Protocol. For example protocol number of ICMP is 1, TCP is 6 and UDP is 17.
• Header Checksum: This field is used to keep checksum value of entire header which is then used to check if the packet is received error-free.
• Source Address: 32-bit address of the Sender (or source) of the packet.
• Destination Address: 32-bit address of the Receiver (or destination) of the packet.
• Options: This is optional field, which is used if the value of IHL is greater than 5. These options may contain values for options such as Security, Record Route, Time Stamp, etc.
Internet Protocol hierarchy contains several classes of IP to be used efficiently in various situations as per the requirement of hosts per network. Broadly, the IPv4 system is divided into five classes of IP Addresses. All the five classes are identified by the first octet of IP.
Internet Corporation for Assigned Names and Numbers is responsible for assigning IP.
The first octet referred here is the left most of all. The octets numbered as follows depicting dotted decimal notation of IP:
The number of networks and the number of hosts per class can be derived by this formula:
When calculating hosts' IP, 2 IP are decreased because they cannot be assigned to hosts, ie the first IP of a network is network number and the last IP is reserved for Broadcast IP.
Class A Address
The first bit of the first octet is always set to 0 (zero). Thus the first octet ranges from 1 – 127, ie
Class A addresses only include IP starting from 1.xxx to 126.xxx only. The IP range 127.xxx is reserved for loopback IP addresses.
The default subnet mask for Class A IP address is 255.0.0.0 which implies that Class A addressing can have 126 networks (27-2) and 16777214 hosts (224-2).
Class A IP address format is thus: 0NNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH
Class B Address
An IP address which belongs to class B has the first two bits in the first octet set to 10, ie
Class B IP range from 128.0.xx to 191.255.xx The default subnet mask for Class B is 255.255.xx
Class B has 16384 (214) Network addresses and 65534 (216-2) Host addresses.
Class B IP format is: 10NNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH
Class C Address
The first octet of Class C IP address has its first 3 bits set to 110, that is:
Class C IP range from 192.0.0.x to 223.255.255.x. The default subnet mask for Class C is 255.255.255.x.
Class C gives 2097152 (221) Network addresses and 254 (28-2) Host addresses.
Class C IP address format is: 110NNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH
Class D Address
Very first four bits of the first octet in Class D IP addresses are set to 1110, giving a range of:
Class D has IP rage from 220.127.116.11 to 18.104.22.168. Class D is reserved for Multicasting. In multicasting data is not destined for a particular host, that is why there is no need to extract host address from the IP address, and Class D does not have any subnet mask.
Class E Address
This IP Class is reserved for experimental purposes only for R&D or Study. IP addresses in this class ranges from 240.0.0.0 to 255.255.255.254. Like Class D, this class too is not equipped with any subnet mask.
Each IP class is equipped with its own default subnet mask which bounds that IP class to have prefixed number of Networks and prefixed number of Hosts per network. Classful IP does not provide any flexibility of having less number of Hosts per Network or more Networks per IP Class.
CIDR or Classless Inter Domain Routing provides the flexibility of borrowing bits of Host part of the IP and using them as Network in Network, called Subnet. By using subnetting, one single Class A IP address can be used to have smaller sub-networks which provides better network management capabilities.
Class A Subnets
In Class A, only the first octet is used as Network identifier and rest of three octets are used to be assigned to Hosts (ie 16777214 Hosts per Network). To make more subnet in Class A, bits from Host part are borrowed and the subnet mask is changed accordingly.
For example, if one MSB (Most Significant Bit) is borrowed from host bits of second octet and added to Network address, it creates two Subnets (21 = 2) with (223-2) 8388606 Hosts per Subnet.
The Subnet mask is changed accordingly to reflect subnetting. Given below is a list of all possible combination of Class A subnets:
In case of subnetting too, the very first and last IP of every subnet is used for Subnet Number and Subnet Broadcast IP respectively. Because these two IP addresses cannot be assigned to hosts, sub-netting cannot be implemented by using more than 30 bits as Network Bits, which provides less than two hosts per subnet.
Class B Subnets
By default, using Classful Networking, 14 bits are used as Network bits providing (214) 16384 Networks and (216-2) 65534 Hosts. Class B IP Addresses can be subnetted the same way as Class A addresses, by borrowing bits from Host bits. Below is given all possible combination of Class B subnetting:
Class C Subnets
Class C IP addresses are normally assigned to a very small size network because it can only have 254 hosts in a network. Given below is a list of all possible combination of subnetted Class B IP address:
Internet Service Providers may face a situation where they need to allocate IP subnets of different sizes as per the requirement of customer. One customer may ask Class C subnet of 3 IP addresses and another may ask for 10 IPs. For an ISP, it is not feasible to divide the IP addresses into fixed size subnets, rather he may want to subnet the subnets in such a way which results in minimum wastage of IP addresses.
For example, an administrator have 192.168.1.0/24 network. The suffix / 24 (pronounced as "slash 24") tells the number of bits used for network address. In this example, the administrator has three different departments with different number of hosts. Sales department has 100 computers, Purchase department has 50 computers, Accounts has 25 computers and Management has 5 computers. In CIDR, the subnets are of fixed size. Using the same methodology the administrator cannot fulfill all the requirements of the network.
The following procedure shows how VLSM can be used in order to allocate department-wise IP addresses as mentioned in the example.
Step – 1
Make a list of Subnets possible.
Step – 2
Sort the requirements of IPs in descending order (Highest to Lowest).
• Sales 100
• Purchase 50
• Accounts 25
• Management 5
Step – 3
Allocate the highest range of IPs to the highest requirement, so let's assign 192.168.1.0 / 25 (255.255.255.128) to the Sales department. This IP subnet with Network number 192.168.1.0 has 126 valid Host IP which satisfy the requirement of the Sales department. The subnet mask used for this subnet has 10000000 as the last octet.
Step – 4
Allocate the next highest range, so let's assign 192.168.1.128 / 26 (255.255.255.192) to the Purchase department. This IP subnet with Network number 192.168.1.128 has 62 valid Host IP Addresses which can be easily assigned to all the PCs of the Purchase department. The subnet mask used has 11,000,000 in the last octet.
Step – 5
Allocate the next highest range, ie Accounts. The requirement of 25 IPs can be fulfilled with 192.168.1.192 / 27 (255.255.255.224) IP subnet, which contains 30 valid host IPs. The network number of Accounts department will be 192.168.1.192. The last octet of subnet mask is 11100000.
Step – 6
Allocate the next highest range range to Management. The Management department contains only 5 computers. The subnet 192.168.1.224 / 29 with the Mask 255.255.255.248 has exactly 6 valid host IP. So this can be assigned to Management. The last octet of the subnet mask will contain 11111000.
By using VLSM, the administrator can subnet the IP subnet in such a way that least number of IP are wasted. Even after assigning IPs to every department, the administrator, in this example, is still left with plenty of IP which was not possible if he has used CIDR.
There are a few reserved IPv4 address spaces which cannot be used on the internet. These addresses serve special purpose and cannot be routed outside the Local Area Network.
Every class of IP, (A, B & C) has some addresses reserved as Private IP addresses. These IPs can be used within a network, campus, company and are private to it. These addresses cannot be routed on the Internet, so packets containing these private addresses are dropped by the Routers.
In order to communicate with the outside world, these IP addresses must have to be translated to some public IP using NAT process, or Web Proxy server can be used.
The sole purpose to create a separate range of private addresses is to control assignment of already-limited IPv4 address pool. By using a private address range within LAN, the requirement of IPv4 addresses has globally decreased significantly. It has also helped delaying the IPv4 address exhaustion.
IP class, while using private address range, can be chosen as per the size and requirement of the organization. Larger organizations may choose class A private IP address range where smaller organizations may opt for class C. These IP addresses can be further sub-netted and assigned to departments within an organization.
The IP range 127.0.0.0 – 127.255.255.255 is reserved for loopback, ie a Host's self-address, also known as localhost address. This loopback IP is completely managed by and within the operating system. Loopback addresses, enable the Server and Client processes on a single system to communicate with each other. When a process creates a packet with destination address as loopback address, the operating system loops it back to itself without having any interference of NIC.
Data sent on loopback is forwarded by the operating system to a virtual network interface within operating system. This address is mostly used for testing purposes like client-server architecture on a single machine. Other than that, if a host machine can successfully ping 127.0.0.1 or any IP from loopback range, implies that the TCP / IP software stack on the machine is successfully loaded and working.
In case a host is not able to acquire an IP from the DHCP server and it has not been assigned any IP manually, the host can assign itself an IP address from a range of reserved Link-local addresses. Link local address ranges from 169.254.0.0 – 169.254.255.255.
Assume a network segment where all systems are configured to acquire IP from a DHCP server connected to the same network segment. If the DHCP server is not available, no host on the segment will be able to communicate to any other. Windows (98 or later), and Mac OS (8.0 or later) supports this functionality of self-configuration of Link-local IP. In absence of DHCP server, every host machine randomly chooses an IP from the above mentioned range and then checks to ascertain by means of ARP, if some other host also has not configured itself with the same IP. Once all hosts are using link local addresses of the same range, they can communicate with each other.
These IP addresses cannot help system to communicate when they do not belong to the same physical or logical segment. These IPs are also not routable.