CCNA - Capitulo 8.2

CCNA - Capitulo 8.2

1. INTERNET LAYER

   • IPV4 - 12 FIELDS - HEADER
     • VERSION (4bits)
       • 4 0100
     • DIFFSERV (DS) - 8 BITS OR 1 bit
       • ENC (2bits)
         • EXPLICIT CONGESTION NOTIFICATION
       • DSCP (6bits)
         • MOST SIGNIFICANT bits OF DiffServ
       • PRIORITY OF EACH PACKET (QoS)
     • HEADER CHEKSUM (16 bits)
       • CORRUPTION PACKET
       • VALIDATE PACKET
     • TIME TO LIVE TTL (8 bits or 1 BYTE )
       • HOPS
         • USED ALSO ICMP
         • EACH HOP THE ROUTER MUST ALSO RECALCULATE THE HEADER CHEKSUM
       • LIMIT THE LIFETIME OF A PACKET
         • DECREMENTS TO ZERO
       • PREVENT A PACKET FROM PASSING THROUGH , AVOIDING LOOP LAYER 3
       • SOURCE DEVICE OF THE IPV4 SETS THE INITIAL TTL
         • IT DEPENDS ON THE OPERATING SYSTEM.
     • PROTOCOL (8 bits )
       • UPPER LAYER PROTOCOL
         • ICMP ( 1 )
         • TCP ( 6 )
         • UDP ( 17 )
       • INDICATES THE PAYLOAD TYPE THAT PACKET IS CARRING
     • INTERNET HEADER LENGHT - IHL (4bits)
       • VALIDAT PACKET
       • VARIABLE LENGHT
         • 60 BYTES
           • WITH FIELD OPTIONS
         • 20 BYTES
           • WITHOUT FIELD OPTIONS
     • FRAGMENT OFFSET (13 bits)
       • PACKET TRACKING
       • ESSENTIAL TO ENSURE THE CORRECT REASSEMBLY OF THE FRAGMENTS OF AN IP PACKET IN THE CORRECT ORDER.
       • REORDER FRAGMENTED PACKET
     • FLAG (3 bits)
       • NULL BIT (RES)
       • DONT' FRAGMENT (DF)
       • MORE FRAGMENT (MF)
       • REORDER FRAGMENTED PACKET
     • SOURCE ADDRESS (32 bits or 4 BYTE )
       • ALWAYS UNICAST
     • TOTAL LENGHT (16 bits or 2 BYTES)
       • TOTAL SIZE OF THE IP PACKET (HEADER + DATA), IN BYTES.
       • VALIDATE PACKET
       • 2 RAISED TO THE POWER OF 16 = 65,535
         • THIS VALUE IS MORE THEORETICAL THAN PRACTICAL. IN REALITY, THE EFFECTIVE LIMIT IS IMPOSED BY THE MTU (MAXIMUM TRANSMISSION UNIT) OF NETWORK INTERFACES.
     • DESTINATION ADDRESS (32 bits or 4 BYTE )
       • UNICAST , MULTICAST , BROADCAST
     • IDENTIFICATION (16 bits or 2 BYTES )
       • REORDER FRAGMENTED PACKAGE
     • DATA
       • 20 BYTES
         • MINIMUM PACKET LAYER 3
       • NOT PART OF THE HEADER
       • to 65536 BYTES
         • THE LIMIT IS DEFINED BY THE MAXIMUM TOTAL HEADER LENGTH FIELD.
       • THE ACTUAL SIZE OF AN IP PACKET IS LIMITED BY THE LINK'S MTU, WHICH IS MUCH SMALLER THAN 65535.
     • OPTIONS
       • 0 TO 40 BYTES
       • OBS : DUE TO THE PRESENCE OF OPTIONS, THE SIZE OF THE DATAGRAM HEADER CAN BE OF VARIABLE LENGTH (20 BYTES TO 60 BYTES).

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