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To create an IP datagram, a host or a router needs to know its own IP address, which is independent of the physical address. The RARP is designed to resolve the address mapping of a machine in which its physical address is known, but its logical (IP) address is unknown. The machine can get its physical address, which is unique locally. It can then use the physical address to get the logical IP address using the RARP protocol. In reality, RARP is a protocol of dynamic mapping in which a given physical address is associated with a logical IP address. To get the IP address, a RARP request is broadcast to all systems on the network. Every host or router on the physical network will receive the RARP request packet, but the RARP server will only answer it as shown in Figure 2.6(b). The server sends a RARP reply packet including the IP address of the requestor.

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IP (logical) addresses are assigned independently from physical (hardware) addresses. The logical address is called a 32-bit IP address, and the physical address is a 48-bit MAC address in Ethernet and token ring protocols. The delivery of a packet to a host or a router requires two levels of addressing, such as logical (IP) address and physical (MAC) addresses. When a host or a router has an IP datagram forwarding to another host or router, it must know the logical IP address of the receiver. Since the IP datagram is encapsulated in a form to be passed through the physical network (such as a LAN), the sender needs the physical MAC address of the receiver. Mapping of an IP address to a physical address can be done by either static or dynamic
mapping. Static mapping means creating a table that associates an IP address with a physical address. But static mapping has some limitations because table lookups are inefficient. As a consequence, static mapping creates a huge overhead on the network. Dynamic mapping can employ a protocol to find the other. Two protocols (ARP and RARP) have been designed to perform dynamic mapping. When a host needs to find the physical address of another host or router on its network, it sends an ARP query packet. The intended recipient recognises its IP address and sends back an ARP response which contains the recipient IP and physical addresses. An ARP request is broadcast to all devices on the network, while an ARP reply is unicast to the host requesting the mapping.


This all about the ARP - Address Resolution Protocol. How its formulation and all. Enjoy..........

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In a connectionless packet delivery system, the basic unit of transfer is the IP datagram. The routing problem is characterised by describing how routers forward IP datagrams and deliver them to their destinations. In a packet switching system, ‘routing’ refers to the process of choosing a path over which to send packets. Unlike routing within a single network, the IP routing must choose the appropriate algorithm for how to send a datagram across multiple physical networks. In fact, routing over the Internet is generally difficult because many computers have multiple physical network connections. To understand IP routing, a TCP/IP architecture should be reviewed completely. The Internet is composed of multiple physical networks interconnected by routers. Each router has direct connections to two or more networks, while a host usually connects directly to one physical network. However, it is possible to have a multihomed host connected directly to multiple network. Packet delivery through a network can be managed at any layer in the OSI stack model. The physical layer is governed by the Media Access Control (MAC) address; the data link layer includes the Logical Link Control (LLC); and the network layer is where most routing takes place. Delivery The delivery of an IP packet to its final destination is accomplished by means of either direct or indirect delivery. Direct delivery occurs when the source and destination of the packet are located on the same physical network. The sender can easily determine whether the delivery is direct or not by extracting the network (IP) address of the destination packet and comparing this address with the addresses of the networks to which it is connected. If a match is found, the delivery is direct. In direct delivery, the sender uses the senders IP address to find the destination physical address. This mapping process can be done by Address Resolution Protocol (ARP). If the destination host is not on the same network as the source host, the packet will be delivered indirectly. In an indirect delivery, the packet goes from router to router through a number of networks until it reaches one that is connected to the same physical network as its final destination. Thus, the last delivery is always a direct delivery, which always occurs after zero or more indirect deliveries. In an indirect delivery, the sender uses the destination IP address and a routing table to find the IP address of the next router to which the packet should be delivered. The sender then uses the ARP to find the physical address of the next router.

This is all description about IP Routing. How it is working. Enjoy..............

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Each IP address is made of two parts in such a way that the netid defines a network and the hostid identifies a host on that network. An IP address is usually written as four decimal integers separated by decimal points i.e. 239.247.135.93. If this IP address changes from decimal-point notation to binary form, it becomes 11101111 11110111 10000111 01011101. Thus, we see that each integer gives the value of one octet (byte) of the IP address. IP addresses are divided into five different classes: A, B, C, D and E. Classes A, B and C differ in the number of hosts allowed per network. Class D is used for multicasting and class E is reserved for future use. Table 2.3 shows the number of networks and hosts in five different IP address classes. Note that the binary numbers in brackets denote class prefixes.


The relationship between IP address classes and dotted decimal numbers is summarised in Table 2.4, which shows the range of values for each class. The use of leading bits as class prefixes means that the class of a computer’s network can be determined by the numerical value of its address. A number of IP addresses have specific meanings. The address 0.0.0.0 is reserved and 224.0.0.0 is left unused. Addresses in the range 10.0.0.0 through to 10.255.255.255 are available for use in private intranets. Addresses in the range 240.0.0.0 through to 255.255.255.255 are class E addresses and are reserved for future use when new protocols are eveloped. Address 255.255.255.255 is the broadcast address, used to reach all systems on a local link


This is the Addressing Schemes description. You please enjoy by taking this information.