The IP layer communicates
using packets called datagrams. Datagrams contain headers
of 20-60 bytes and data payloads of up to 65K bytes.
Datagrams contain the basic IP source and destination addresses,
where the IP (Internet Protocol) address consists of four bytes
as in 188.8.131.52.
The left most byte is the highest order addressing and so could
represent a region or country. The lower order bytes narrow the
address down until the final byte indicates, for example, a particular
computer on a LAN (Local Area Network).
There are a number of special addresses. For example,
any address beginning with 127 such as 127.0.0.1 will translate
as a loop back address. This means that any packets sent
with this destination address will automatically return to the
Users typically deal with text addresses called
hostnames that are easier to remember than the numerical addresses.
For example, java.sun.com is a hostname. Special nodes on the
Internet called Domain Name Servers (DNS) translate hostnames
into the corresponding numerical IP addresses. A hostname is composed
of a top level domain such as com,
These domains are then divided into second level subdomains such
The systems at these locations can divide a domain further such
Transmission Control Protocol (TCP) and the User
Datagram Protocol (UDP) are two separate Internet protocols within
the Transport Layer.
Datagrams from the same message can travel completely
different paths as the routers dynamically choose paths for the
same destination address so as to avoid loading down any one link.
Thus datagrams may become lost or arrive out of order from how
they were sent.
The Transport Layer attempts to smooth over the
problems of the IP layer. The Transport Layer can place packets
into the proper order and request retransmission of missing
packets (that is, packets not arriving within a given time).
TCP guarantees bytes in correct order. UDP does not guarantee
correct order but has lower overhead than TCP.
For some applications, such as audio and video transmission,
the loss of a few bytes is not noticeable and so the UDP serves
very well since it is faster than TCP.
The Application Layer involves
all the user related programs like web browsers and ftp. The application
layer programs use various protocols:
- Hyper Text Transfer Protocol
- FTP -
File Transfer Protocol
- Telnet -
the telnet protocol provides for console sessions.
- Simple Mail Transport
Protocol - email.
These protocols require TCP since data can not be randomly dropped
from source files and web pages. Other application layer programs
which sends test packets to a given IP address, can use the simpler
UDP. Most Java network programs only deal with this application
References & Web