I have some doubts over physical and logical address system. I am reading Fourouzan TCP/IP protocol suite.By reading first chapter what I understood is that to communicate between two different networks the logical address communicate with each other .They are provided by ISP. So are they always unique with the router in my home..?? I mean if I apply for an Internet connection does my router have unique IP or logical address ..?? or is it that there is a more centralized system of my area which has a unique IP which in turn again distributes IP to router in my home..??? And since I do not come to know about physical address of source how does the source IP will determine that exactly who is my destination logical address .since my computer will get assigned with new IP everytime I login in a particular network .I am pretty confused about how it works..??? Thanks.
In your business, you may have multiple networks. You may have a single site with multiple VLANs, and each VLAN has a different network, or you may have multiple sites, and each site has a different network, possibly multiple networks, per site.
The physical addresses are usually MAC addresses burned into the NICs of the hosts connected to the networks. Layer-2 uses those addresses to deliver frames from one host to another host on the same network.
The logical addresses are assigned to layer-3, usually through DHCP, but there are other methods, too. The logical addresses on one network will all be in the same network. Hosts will use the layer-3 addresses to communicate with hosts on a different network.
A host sending something out will look at the layer-3 address of the destination, and it will compare that to its own layer-3 address.
if the addresses are on the same network, the host will create a frame with the destination layer-2 address of the destination host, and the frame will be sent directly to the destination host.
If the destination host is on a different network, the host will create the frame with the layer-2 address of its configured gateway, and the frame will be sent directly to the gateway. The gateway then must find a path to the network where the destination host is, using the layer-3 address, and the gateway will forward the packet toward the destination network.
The layer-2 (physical) addresses on the frames the host sends will not survive from one network to the next. The layer-2 addresses are in the frame headers, and a layer-3 device (gateway/router) will strip the frame from the packet and discard it in order to get to the layer-3 (logical) packet with the layer-3 addresses. The router will build a new frame for the new interface toward the destination network to which it forwards the packet. This may happen several time before the packet reaches the destination network.
I believe your question boils down to, How does Internet Protocol addressing work? Wikipedia has a great intro:
The IP address space is managed globally by the Internet Assigned Numbers Authority (IANA), and by five regional Internet registries (RIR) responsible in their designated territories for assignment to end users and local Internet registries, such as Internet service providers. Addresses have been distributed by IANA to the RIRs in blocks of approximately 16.8 million addresses each. Each ISP or private network administrator assigns an IP address to each device connected to its network. Such assignments may be on a static (fixed or permanent) or dynamic basis, depending on its software and practices.