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In the preceding topic I described
the general operation of IP and boiled down its primary job as internetwork
datagram delivery. I also explained the most important characteristics
of how IP does this job. With that as a foundation, let's now look a
bit deeper, at how IP gets the job done. A
good way to do this is to examine the various functions that the Internet
The exact number of IP functions
depends on where you draw the line between certain activities.
For explanatory purposes, however, I view IP as having four basic functions
(or more accurately, function sets):
- Addressing: In order to perform the job
of delivering datagrams, IP must know where to deliver them to! For
this reason, IP includes a mechanism for host addressing. Furthermore,
since IP operates over internetworks, its system is designed to allow
unique addressing of devices across arbitrarily large networks. It also
contains a structure to facilitate the routing of datagrams to distant
networks if that is required.
Since most of the other TCP/IP protocols use IP, understanding
the IP addressing scheme is of vital importance
to comprehending much of what goes on in TCP/IP.
- Data Encapsulation and Formatting/Packaging:
As the TCP/IP network layer protocol, IP accepts data from the
transport layer protocols UDP and TCP.
It then encapsulates
this data into an IP datagram using a special format
prior to transmission.
- Fragmentation and Reassembly: IP datagrams
are passed down to the data link layer for transmission on the local
network. However, the maximum frame size of each physical/data-link
network using IP may be different. For this reason, IP includes the
ability to fragment IP datagrams into pieces so they can each
be carried on the local network. The receiving device uses the reassembly
function to recreate the whole IP datagram again.
Note: Some people view fragmentation and reassembly as distinct functions, though clearly they are complementary and I view them as being part of the same function.
- Routing / Indirect Delivery: When an IP
datagram must be sent to a destination on the same local network, this
can be done easily using the network's underlying LAN/WLAN/WAN protocol
using what is sometimes called direct delivery. However, in many
(if not most cases) the final destination is on a distant network not
directly attached to the source. In this situation the datagram must
be delivered indirectly. This is accomplished by routing
the datagram through intermediate devices
(shockingly called routers). IP accomplishes this in concert
with support from the other protocols including ICMP
and the TCP/IP
gateway/routing protocols such as RIP
As you continue on in this section
on IP will find that I have structured the sub-sections that provide
more detail one the main IP version and IP-related protocols based on
these general functions.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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