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Internet, Packets and Routers
(9.3.1 part 1) (Hops (Each time a router…
Internet, Packets and Routers
(9.3.1 part 1)
Structure of Internet
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The major Internet communication links form the Internet Back bone - Network Service Providers (NSPs) eg BT
Internet Service Provider - we connect via these, which then connect to the NSPs.
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Packet
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As well as the data to be transmitted, a packet includes the sender and receiver IP addresses, the package sequence number(for re-assembly) (Header) and an error check (eg checksum) (Tail)
Checksum - bit count of the packet when sent. Recalculated at destination to check transmission is complete. (But see Hops)
Router
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Uses forwarding tables as well as using TCP/IP to communicate with other routers to work out the best path for each packet
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Routers operate at the Network Layer level of the TCP/IP model, storing logical IP addresses.
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Networks and Subnets
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Each subnet is assigned a network address,
as is each node within it
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Hops
Each time a router receives a packet,
it subtracts 1 from the Hop Limit.
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The router discards any packets received
with a zero Hop Limit, thus preventing
'lost' packets endlessly clogging the Internet.
Decrementing the Hop Limit changes the number of bits in the packet, so Routers update the packet's Checksum
A ping can be used to see how many hops
it takes to get between 2 hosts.
This can be used to help diagnose network errors.
If a router deletes a package, it sends an
Internet Control Message Protocol (ICMP) message
back to the sender, which may trigger a resend
The Time to Live(TTL) (IPv4) or Hop Limit (IPv6) can be set between 1 and 255 by the operating system
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