Network addressing

Network Addressing

IP address Structure

  • An IP address is simply a series of 32 binary bits. The 32 bits are grouped into four 8-bit bytes called octets. To make the IP address easier to understand, each octet is presented as its decimal value, separated by a decimal point or period.

  • When a host is configured with an IP address, it is entered as a dotted decimal number such as Imagine if you had to enter the 32-bit binary equivalent of this – 11000000101010000000000100000101. If just one bit was mistyped, the address would be different and the host may not be able to communicate on the network.

  • The 32-bit IP address is defined with IP version 4 (IPv4) and is currently the most common form of IP address on the Internet. There are over 4 billion possible IP addresses using a 32-bit addressing scheme.

Parts of IP address

  • The logical 32-bit IP address is hierarchical and is made up of two parts. The first part identifies the network and the second part identifies a host on that network. Both parts are required in an IP address.

  • As an example, if a host has IP address the first three octets, (192.168.18), identify the network portion of the address, and the last octet, (57) identifies the host. This is known as hierarchical addressing because the network portion indicates the network on which each unique host address is located. Routers only need to know how to reach each network, rather than needing to know the location of each individual host.

Relation b/w IP Address and Subnet Mask

  • There are two parts to every IP address. When an IP host is configured, a subnet mask is assigned along with an IP address. Like the IP address, the subnet mask is 32 bits long. The subnet mask signifies which part of the IP address is network and which part is host.

  • The subnet mask is compared to the IP address from left to right, bit for bit. The 1s in the subnet mask represent the network portion; the 0s represent the host portion

  • When a host sends a packet, it compares its subnet mask to its own IP address and the destination IP address. If the network bits match, both the source and destination host are on the same network and the packet can be delivered locally. If they do not match, the sending host forwards the packet to the local router interface to be sent on to the other network.

Types of IP Address

Public and Private Addresses

All hosts that connect directly to the Internet require a unique public IP address. Because of the finite number of 32-bit addresses available, there is a risk of running out of IP addresses. One solution to this problem was to reserve some private addresses for use exclusively inside an organization. This allows hosts within an organization to communicate with one another without the need of a unique public IP address.

Static and Dynamic Address Assignment

IP addresses can be assigned either statically or dynamically.


  • With a static assignment, the network administrator must manually configure the network information for a host. At a minimum, this includes the host IP address, subnet mask and default gateway.

  • Static addresses have some advantages. For instance, they are useful for printers, servers and other networking devices that need to be accessible to clients on the network. If hosts normally access a server at a particular IP address, it would not be good if that address changed.

  • Static assignment of addressing information can provide increased control of network resources, but it can be time consuming to enter the information on each host. When entering IP addresses statically, the host only performs basic error checks on the IP address. Therefore, errors are more likely to occur.


  • On local networks it is often the case that the user population changes frequently. New users arrive with laptops and need a connection. Others have new workstations that need to be connected. Rather than have the network administrator assign IP addresses for each workstation, it is easier to have IP addresses assigned automatically. This is done using a protocol known as Dynamic Host Configuration Protocol (DHCP).

  • DHCP provides a mechanism for the automatic assignment of addressing information such as IP address, subnet mask, default gateway, and other configuration information.

  • Benefit of DHCP is that an address is not permanently assigned to a host but is only leased for a period of time. If the host is powered down or taken off the network, the address is returned to the pool for reuse. This is especially helpful with mobile users that come and go on a network.

Configuring DHCP


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