Networking —

Understanding How Security Affects a Network

Implementing security measures can have a significant impact on the network. How much of an impact it has depends on which security measures are implemented and the habits of the network users. Several security measures are used on networks including port blocking, authentication schemes, encryption, and so on. While in today's world we may have no choice but to implement these measures, as a network administrator, you'll need to be aware how they impact the overall network.

Blocking Port Numbers

Port blocking is one of the most widely used security methods on networks. Port blocking is associated with firewalls and proxy servers, although it can be implemented on any system that provides a means to manage network data flow, according to data type.

Essentially, when you block a port, you disable the ability for traffic to pass through that port, thereby filtering the traffic. Port blocking is typically implemented to prevent users on a public network from accessing systems on a private network, although it is equally possible to block internal users from external services, and internal users from other internal users, by using the same procedure.

Depending on the type of firewall system in use on a network, you might find that all the ports are disabled (blocked) and that the ones you need traffic to flow through must be opened. The benefit of this strategy is that it forces the administrator to choose the ports that should be unblocked rather than specify those that need to be blocked. This ensures that you allow only those services that are absolutely necessary into the network.

What ports remain open largely depends on the needs of the organization. For example, the ports associated with the services listed in Table 1 are commonly left open.

Table 1 Commonly Opened Port Numbers and Their Associated Uses

Port Number

Protocol

Purpose

21

FTP

File transfers

22

SSH

Secure remote sessions

25

SMTP

Email sending

53

DNS

Hostname resolution

80

HTTP

Web browsing

110

POP3

Email retrieval

123

NTP

Time information

161

SNMP

Network Management

443

HTTPS

Secure Web transactions

3389

RDP

Windows Terminal Services or Windows Remote Desktop

These are, of course, only a few of the services you might need on a network, and allowing traffic from other services to traverse a firewall is as easy as opening the port. Keep in mind, though, that the more ports that are open, the more vulnerable you become to outside attacks. You should never open a port on a firewall unless you are absolutely sure that you need to.

Port Blocking and Network Users

Before you implement port blocking, you should have a very good idea of what the port is used for. Although it is true that blocking unused ports does not have any impact on internal network users, if the wrong port is blocked, you can create connectivity issues for users on the network.

For instance, imagine that a network administrator was given the task of reducing the amount of spam emails received by his company. He decided to block port 25, the port used by the Simple Mail Transfer Protocol (SMTP). He may have succeeded in blocking the spam email, but in the process, he also prevented users from sending email.