Subnetting

Subnetting is another method of managing IP addresses. This method of dividing full network address classes into smaller pieces Has prevented complete IP address exhaustion. It is impossible to cover TCP/IP without mentioning subnetting. It is not always necessary to subnet a small network. However, for large or extremely large networks, subnetting is required.

Subnetting a network means to use the subnet mask to divide the network and break a large network up into smaller, more efficient and manageable segments, or subnets. With subnetting, the network is not limited to the default Class A, B, or C network masks and there is more flexibility in the network design.

Subnet addresses include the network portion, plus a subnet field and a host field. The subnet field and the host field are created from the original host portion for the entire network.

To create a subnet address, a network administrator borrows bits from the host field and designates them as the subnet field. The minimum number of bits that can be borrowed is two. When creating a subnet, where only one bit was borrowed the network number would be the .0 network. The broadcast number would then be the .255 network. The maximum number of bits that can be borrowed can be any number that leaves at least two bits remaining, for the host number.

Addresses in a network with and without subnetting

Subnet mask

Example Subnet mask 255.255.255.192 will create 4 subnet

Many hosts in different networks may use the same private space addresses. Packets using these addresses as the source or destination should not appear on the public Internet. The router or firewall device at the perimeter of these private networks must block or translate these addresses. Even if these packets were to make their way to the Internet, the routers would not have routes to forward them to the appropriate private network.

Who hold the address?

Range NAT (Network Address Translation) Private addresses (others are called Public IP or Global address) : Range Total 10.0.0.0 to 10.255.255.255 224 172.16.0.0 to 172.31.255.255 220 192.168.0.0 to 192.168.255.255 216

NAT can be implemented on a device at the edge of the private network. NAT allows the hosts in the network to "borrow" a public address for communicating to outside networks. While there are some limitations and performance issues with NAT, clients for most applications can access services over the Internet without noticeable problems.

Address translation

Translation Using One IP address Another types of translations are : Using a Pool of IP address : several private hosts can communicate with the same external host at the same time Using both IP address and Port Numbers  PAT (Port address translation)

Private Address Private Port External Address External Port Using both IP address and Port Numbers e.g. Five-column translation table Private Address Private Port External Address External Port Transport Protocol 172.18.3.1 1400 25.8.3.2 80 TCP 172.18.3.2 1401 ... Reduce the ambiguity of the previous type of translation.

ARP (Address Resolution Protocol) Know the IP, ask hardware address (MAC address) Mapping a logical address with the physical layer

ARP packet

Mapping Physical address to Logical address RARP BOOTP DHCP

Mapping Physical address to Logical address RARP BOOTP DHCP

RARP Reverse Address Resolution Protocol (RARP) associates a known MAC addresses with an IP addresses. This association allows network devices to encapsulate data before sending the data out on the network. A network device, such as a diskless workstation, might know its MAC address but not its IP address. RARP allows the device to make a request to learn its IP address. Devices using RARP require that a RARP server be present on the network to answer RARP requests.

the source initiates a process called a RARP request the source initiates a process called a RARP request. This request helps the source device detect its own IP address. RARP requests are broadcast onto the LAN (within a network) and are responded to by the RARP server which is usually a router.

In RARP request (sent by the workstation which needs an IP address): Operation code : 3 Source MAC address : its own MAC address Destination MAC address : FF:FF:FF:FF:FF:FF (makes an RARP request broadcast) Source IP address : undefined Destination IP address : undefined In RARP reply (sent by an RARP server): Operation code : four Source MAC address : MAC address of the RARP server Destination MAC address : MAC address of the workstation that sends the RARP request Source IP address : IP address of the RARP server Destination IP address :given IP address

BOOTP The bootstrap protocol (BOOTP) operates in a client-server environment and only requires a single packet exchange to obtain IP information.   However, unlike RARP, BOOTP packets can include the IP address, as well as the address of a router, the address of a server, and vendor-specific information. One problem with BOOTP, however, is that it was not designed to provide dynamic address assignment.

In BOOTP request (sent by the workstation which needs an IP address): Source MAC address : its own MAC address Destination MAC address : FF:FF:FF:FF:FF:FF (makes an BOOTP request broadcast) Source IP address : undefined Destination IP address : 255.255.255.255 In BOOTP reply (sent by an BOOTP server): Source MAC address : MAC address of the BOOTP server Destination MAC address : MAC address of workstation sends the BOOTP request Source IP address : - IP address of the BOOTP server Destination IP address :- given IP address - IP broadcast

DHCP Dynamic host configuration protocol (DHCP) is the successor to BOOTP. Unlike BOOTP, DHCP allows a host to obtain an IP address dynamically without the network administrator having to set up an individual profile for each device. All that is required when using DHCP is a defined range of IP addresses on a DHCP server. As hosts come online, they contact the DHCP server and request an address.

The DHCP server chooses an address and leases it to that host The DHCP server chooses an address and leases it to that host. With DHCP, the entire network configuration of a computer can be obtained in one message. The major advantage that DHCP has over BOOTP is that it allows users to be mobile. The importance to this DHCP advancement is its ability to lease an IP address to a device and then reclaim that IP address for another user after the first user releases it.

In DHCP request (sent by the workstation which needs an IP address): Source MAC address : its own MAC address Destination MAC address : FF:FF:FF:FF:FF:FF (makes an DHCP request broadcast) Source IP address : undefined Destination IP address : 255.255.255.255 In DHCP reply (sent by an DHCP server): Source MAC address : MAC address of the DHCP server Destination MAC address : MAC address of workstation sends the BOOTP request Source IP address : - IP address of the DHCP server Destination IP address :- given IP address - IP broadcast Gateway information

Comparison RARP: BOOTP : DHCP RARP server and its client are in each network Static assignment of the IP address and manually configuration by admin RARP request is broadcast BOOTP : Application-layer process The BOOTP server and its client can be in different networks with an aid of a relay agent Still static assignment of IP address and manually configuration DHCP Provide both static address allocation and dynamic address allocation Automatically assignment of IP address and configuration Mobile user can be supported