Advanced computer Networks Notes-3

This article covers the following topics:ethernet frame format,extending ethernet with bridges,classful internet address.

Ethernet Frame Format

• Data is transmitted as a frame.



• Ethernet frames are of variable length,with no frame smaller than 64 octets or larger than 1518 octets.









• The various fields in a frame are:
  
  
  
  • Preamble:Consists of 64 bits of alternating 1's and 0's to help receiving interfaces synchronize.
  
  
  
  • Destination address:It consists of 48 bits destination ethernet address.
  
  
  • Source address:It consists of 48 bits sender ethernet address.
  
  
  • Frame type:Contains 16 bit integer which identifies type of data carried by the frame.
  
  
  • Frame data:It may be 46-1500 octets in length and contains the frame data.
  
  
  • Cyclic redundancy check:It is 32 bits in length and helps in error detection.
  
  
  
  
  


• Since the frame type field specifies the type of data the frame contains it is called self identifying.Thus helps the sender use the necessary protocol depending on the frame type.

Extending an Ethernet with Bridges

• Ethernet cable has a maximum length,so repeaters and bridges are used to extend the network.



• Repeaters deals with signal.Thus noise,interference also transmitted by repeaters.


• Bridges are more efficient since it deals with packets thus not transmitting noise,interference.


• Bridges hide the details of interconnection:set of bridged elements acts like a single ethernet.


• Bridges are considered as transparent since computers do not know how many bridges have been used.


• Adaptive or learning bridges:
  
  
  
  
  • Adaptive bridge consists of computers with two ethernet interfaces.
  
  
  • The software in adaptive bridge keeps two address lists one for each interface.
  
  
  • When a frame arrives from Ethernet E₁,the adaptive bridge add 48 bit source address to the list associated with E₁ and vice versa.
  
  
  • Overtime bridge learns which systems lie on E₁ and E₂.
  
  
  
  • After recording the source address of the frame,the adaptive bridge uses the destination address to determine whether to forward this frame.
  
  
  • If the address list shows that the destination lies on the Ethernet from where the frame arrived it won't transmit the frame otherwise it will tramsmit the frame to the next Ethernet.
  
  
  • An adaptive Ethernet bridge connects two Ethernet segments, forwarding frames from one to the other. It uses source addresses to learn which machines lie on which Ethernet segment, and it combines information learned with destination addresses to eliminate forwarding when unnecessary.
  
  
  • From TCP/IP point of view:
    
    Because the connection among physical cables provided by bridgesand repeaters is transparent to machines using the Ethernet, we think of multiple Ethernet segments connected by bridges and repeaters as a single physical network system.
    
  
  
  

Classful Internet address

The original classful addressing scheme

• Ethernet address is used to identify individual systems connected to an ethernet.


• Whereas internet address or IP address is used to uniquely identify systems connected to internet.



• Internet is considered as a large network like anyother physical network and it is a virtual design imagined by its designers and implemented entirely in software.


• Each host on internet is assigned a unique 32 bit address called the internet address or IP address that is uswd in all communication with that host.


• IP address consists of a netid and a postid portion where netid identifies the network and postid identifies the host connected to that network.








• IP address is self identifying because boundary between prefix ans suffix can be computed from address alone.


• The five forms of IP addresses used in with the original classful addressing scheme are:
  
  
  
  • Class A addresses: Used by networks that have more than 2¹⁶ hosts.Devotes 7 bits to netid and 24 bits to hostid.
  
  
  
  • Class B addresses:Used for intermediate size networks that have between 2⁸ and 2¹⁶ hosts,allocate 14 bits to netid and 16 bits to hostid.
  
  
  • Class C addresses:Used for networks having less than 2⁸ hosts,allocate 21 bits to netid and 8 bits to hostid.
  
  
  
  
  


• Suppose if a system is connected to two jnetworks then it would have 2 IP addresses for each networks and such hosts are called multi-homed hosts.


• Because IP addresses encode both network and a host on that network,they do not specify an individual computer,but a connection to a network.


• All bits in hostid 0's indicate the network itself.


• All bits in hostid 1's indicate directed broadcast which means the packet send shall be accepted by all systems in the network.



• All 32 bits (hostid and netid) 1's indicate limited broadcast host adopts this during startup when it doesn't know its IP address.