Both TCP and UDP require port numbers in order to communicate with the upper layers.  These port numbers are used to keep track of varying conversations which criss-cross the network simultaneously. The origin port numbers are dynamically assigned by the source host, most of them will be  at some number above 1024.   All the numbers below 1024 are reserved for specific services as defined in RFC 1700 – they are known as well known port numbers.

Any virtual circuit which is not assigned with a specified service will always be assigned a random port number from this range above 1024.    The port numbers will identify the source and destination in the TCP segment.    Here’s some common port numbers that are associated with well known services:

  • FTP – 21
  • Telnet -23
  • DNS – 53
  • TFTP – 69
  • POP3 – 110
  • News – 144

As you can see all the port numbers assigned are under 1023, whereas above 1024 and above are assigned by the upper layers to set up connections with other hosts.

The internet layer exists for two main reasons, routing and providing a specific network interface to the upper layers. As regards to routing none of the upper or lower layer protocols have any specific functions. Al the routing functionality is primarily the job of the internet layer. As well as routing the internet layer has a second function – to provide a single network interface and gateway to the upper layer protocols.
Application programmers, use this layer to to access the functionality into their application for network access. It is important as it ensures that there is a standardization to access the network layer. Therefore the same functions apply whether you’re on a ethernet or Token ring network.

IP provides a single network interface to access all of these upper layer protocols. The following protocols specifically work at the internet layer:

  • Internet Protocol (IP)
  • Internet Protocol (ICMP)
  • Address Resolution Protocol (ARP)
  • Reverse Address Resolution Protocol (RARP)

The internet protocol is essentially the Internet layer, all the other protocols merely support this functionality. So if for instance you buy UK proxy connections then IP would look at each packet’s address. Then using a routing table, the protocol would decide where the packet should be routed next. The other protocols, the network access layer ones at the bottom of the OSI model are not able to see the entire network topology as they only have connections to the physical addresses.

In order to decide on the specific route, the IP layer needs to answer two specific questions,. The first is which network is the destination host on and the second is what is the ID on that network.   these can be determined and allocated as the logical and hardware address.  The logical address is better known as the IP address and is a unique identifier on any network of the location of a specific host.  These are allocated by specific location and are used by websites to determine resources, so for example to watch BBC iPlayer in Ireland you’d need to route through a UK IP address and not your assigned Irish address.


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