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Ports
The IP address is the main identifier used to specify a destination to send packets to within a network. But since IP compatible
devices can make simultaneous connections for different reasons (e.g. web surfing and email), a scheme is used to designate
a specific “port” on a machine, which is essentially a 16-bit sub-address contained within the header of the packet. These
ports are usually written as simple decimal values (e.g. 80, 5060), and traffic sent to a specific port on a machine can only
be accessed by a program or service “listening” on that port.
TCP vs. UDP
The most common types of IP traffic fall in two sub-categories, TCP/IP and UDP/IP. The difference is important. Most web-
related traffic travels via TCP, which has built in mechanisms for integrity checking and error-correction. This means that if the
TCP “stack” within a machine has delivered a packet from the network, the packet is guaranteed to be correct, and if lost will
be resent.It might surprise you to know that it’s not TCP that’s used for most real-time media on the web. This is because TCP
has quite a bit of overhead in terms of data, and can easily add time delays if packets get corrupted.
VoIP and other real-time communication protocols use UDP, which is a much simpler delivery method. There is no error
correction or resending available at the native UDP layer. UDP is sometimes referred to as the “send and pray” method,
since the network provides no guarantees of delivery of any kind. In it’s simplicity, UDP is a better choice for real-time
communications because higher-level applications can be designed to make smart choices about error protection vs. delay.
Packets sent on IP networks will include a destination IP address/port combination, and a source IP address/port combination.
These act like the destination and return address on an envelope, and allow the packets to be responded to over the network.
The destination port is the most important to IT people, as it’s the one that they need to be sure is open to receiving
communications. When IT folks refer to a service as “running on port x” they are referring to the destination port.
We designate an IP connection via its protocol, destination IP address, and port combination in this form:
<protocol>
<destination address:port>
e.g.
UDP 192.168.0.7:5060
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