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.
Routing IP Address – How Logical Address Works
ip addressrouting
Related Solutions
You may have already learned that network addresses (routes) can be summarized. That is, many network addresses can be summarized (represented) by a single network address (i.e a route). A default route can be considered a summary route that contains all network addresses. Because routes can be summarized, routers may not have complete information about the entire internet, but they may have a route to a large summary network that contains the destination. That is, they know enough to forward the packet to another router that will have more information.
So if you send me data, your provider may not know where my IP address is located. But it may know that my IP is in the range of addresses that are located in the United States. So all your ISP needs to know is how to forward it to a router closer to the US. That router will have more information to get the packet closer to me. It in turn will forward to another router with more specific information, until the data gets to me.
The Tier 1 and Tier 2 providers that make up the backbone of the Internet have routers that carry routes to all the networks connected to the Internet (currently more than 500,000 routes). Your IP packet will eventually be forwarded to a router that does know to get to the destination network.
First off - your English appears to me to be excellent. Secondly your summation of bridging (question 1) and consequently your understanding of the subject are very good. I would probably word some of that slightly different but your summation is fair and it appears that your understanding is good.
Let's make one assumption at this point to keep things simple - this is all Ethernet and IP.
Having said that there are two steps that I would add to your bridging summation that might clear up your understanding of routing (question 2) a little:
- when your network stack is handed data to transmit there is a route table look up done (every device on an IP network contains a route table - to see it from a command prompt - on linux type "route", on windows type "route print".) to determine two things:
a) which interface to transmit the data out
b) if the destination is directly reachable or is remote (beyond a router).
- Once the route is determined the network stack will ARP for a MAC address.
So when Alice is sending data to Charlie she does a route lookup - it is determined that his machine is directly accessible on the LAN. She then ARPs for his MAC address. Once the route has been determined and the MAC address has been acquired the network stack can then assemble the packet for transmission. This particular conversation as you mentioned only traverses switch A as it inspects the destination MAC.
Now that we have fleshed out question one a little better let's apply some of these concepts to question 2.
When Alice desires to send data to Bob the first step is a route lookup. It is determined that Bob is not on the LAN. Thus the route to Bob is the default route (assuming Router A is configured as such). (As your book points out Router A - and by extension Alice - don't know where Bob is. They only know the general direction - or "route".) So Alice ARPs for Router A. Then assembles the packet with destination IP address(network layer) of Bob and MAC address(datalink layer) of Router A. Upon receiving this packet Router A 1)strips off the MAC address(datalink layer) of Alice 2)does a route lookup 3)ARPs for router B 4)then reassembles the packet with MAC address of Router B. Router B repeats these steps ARPing for Bob and forwards the packet.
So the network layer is assembled at the start - source Alice and destination Bob - and never changes. The datalink layer on the other hand is deleted and reassembled each and every leg of the trip.
Best Answer
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.