CHAPTER 5

A host needs an IP address to participate on the Internet.

IP address

Is a logical network address that identifies a particular host. It must be properly configured and unique in order to communicate with other devices on the Internet.

Is assigned to the Network interface connection for a host.
This connection is usually a network interface card (NIC) installed in the device.

Examples:
workstations, servers, network printers and IP phones.
Some servers can have more than one NIC and each of these has its own IP address.
Router interfaces that provide connections to an IP network will also have an IP address.

Is simply a series of 32 binary bits (ones and zeros)

The 32 bits are grouped into four 8-bit bytes called octets.

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.

If all 8 bits are 1s, 11111111 the value of the octet is 255 (128+64+32+16+8+4+2+1).

If the 8 bits are mixed, such as the example

Hierarchical addressing

The logical 32-bit IP address 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.

Routers only need to know how to reach each network.

subnet mask
signifies which part of the IP address is network and which part is host.

The 1s in the subnet mask represent the network; the 0s represent the host.

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.

IP addresses classes

Classes A, B and C are commercial addresses and are assigned to hosts. Class D is reserved for multicast use and Class E is for experimental use.

Class C addresses have three octets for the network portion and one for the hosts. Usually assigned to small networks.

Class B addresses have two octets to represent the network portion and two for the hosts. Typically used for medium-sized networks.

Class A addresses have only one octet to represent the network portion and three to represent the hosts. Typically assigned to large organizations.

Unique public IP address
All hosts that connect directly to the Internet needs it.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.

RFC 1918
Is a standard that reserves several ranges of addresses within each of the classes A, B and C.



Private addressesAre not routed on the Internet
The use of private addresses can provide a measure of security since they are only visible on the local network.
Loopback addressThere are also private Addresses that can be used for the diagnostic testing of devices.
The class A, 127.0.0.0 network, is reserved for it.

we also categorize IP addresses as
UNICAST
For a unicast packet to be sent and received, a destination IP address must be in the IP packet header. A corresponding destination MAC address must also be present in the Ethernet frame header. The IP address and MAC address combine to deliver data to one specific destination host.

BROADCAST

This means that all hosts on that local network (broadcast domain) will receive and look at the packet. Many network protocols, such as ARP and DHCP use broadcasts.

A broadcast IP address for a network needs a corresponding broadcast MAC address in the Ethernet frame.

MULTICAST

As with a unicast or broadcast address, multicast IP addresses need a corresponding multicast MAC address to actually deliver frames on a local network. The multicast MAC address is a special value that begins with 01-00-5E in hexadecimal. The value ends by converting the lower 23 bits of the IP multicast group address into the remaining 6 hexadecimal characters of the Ethernet address.

IP addresses can be assigned either statically or dynamically


STATIC

With a static assignment, the network administrator must manually configure the network information for a host.
This includes:
the host IP address, subnet mask and default gateway.

Advantages:
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.

Provide increased control of network resources, but it can be time consuming to enter the information on each host. Therefore, errors are more likely to occur.

It is important to maintain an accurate list of which IP addresses are assigned to which devices.These are permanent addresses and are not normally reused.


DYNAMIC

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:
an address is not permanently assigned to a host but is only leased for a period of time.

The router provides a GATEWAY through which hosts on one network can communicate with hosts on different networks. Each interface on a router is connected to a separate network.


Every host on a network must use the router as a gateway to other networks. Therefore, each host must know the IP address of the router interface connected to the network where the host is attached. This address is known as the default gateway address It can be either statically configured on the host, or received dynamically by DHCP.

When an integrated router is configured as a DHCP server, it provides its own internal IP address as the default gateway to DHCP clients. It also provides them with their respective IP address and subnet mask.


The integrated router acts as a DHCP server for all local hosts attached to it, either by Ethernet cable or wirelessly.The default IP address configured on the local integrated router interface is usually a private Class C address.

When an integrated router is connected to the ISP, it acts like a DHCP client to receive the correct external network IP address for the Internet interface. ISPs usually provide an Internet-routable address, which enables hosts connected to the integrated router to have access to the Internet.


WAYS OF CONNECTING A HOST

Direct Connection
Some customers have just a single computer with a direct connection from the ISP through a modem. In this case, the public address from the ISP DHCP server is assigned to the single host.

Connection Through an Integrated Router

When there is more than one host that needs access to the Internet, the ISP modem can be attached directly to an integrated router instead of directly to a single computer. This enables the creation of a home or small business network. The integrated router receives the public address from the ISP. Internal hosts receive private addresses from the integrated router.

Connection Through a Gateway Device
Gateway devices combine an integrated router and a modem in one unit, and connect directly to the ISP service. As with integrated routers, the gateway device receives a public address from the ISP and internal PCs will receive private addresses from the gateway device.

TRANSLATION NAT
The integrated router is able to translate many internal IP addresses to the same public address, by using it.Only packets destined for other networks need to be translated.