Network Devices..

Network Devices

Some network devices (repeaters, bridges and switches) were discussed in the previous sections. These are all dedicated hardware devices. Network devices can also be non-dedicated systems running network software.

Routers

A router is a hardware device that connects two or more networks. Routers are the primary backbone device of the Internet, connecting different network technologies into a seamless whole. Each router is assigned two or more IP addresses because each IP address contains a prefix that specifies a physical network. Before a packet is passed to the routing software, it is examined. If it is corrupted, it is discarded. If it is not corrupted, a routing table is consulted to determine where to send it next. By default, routers do not propagate broadcast packets . A router can be configured to pass certain types of broadcasts.

Firewalls

A firewall is a computer, router, or some other communications device that controls data flow between networks. Generally, a firewall is a first-line defense against attacks from the outside world. A firewall can be hardware-based or software-based. A hardware-based firewall is a special router with additional filter and management capabilities. A software-based firewall runs on top of the operating system and turns a PC into a firewall.
Conceptually, firewalls can be categorized as Network layer (aka Data Link layer) or Application layer. Network layer firewalls tend to be very fast. They control traffic based on the source and destination addresses and port numbers, using this information to decide whether to pass the data on or discard it. Application layer firewalls do not allow traffic to flow directly between networks. They are typically hosts running proxy servers. Proxy servers can implement protocol specific security because they understand the application protocol being used. For instance, an application layer firewall can be configured to allow only e-mail into and out of the local network it protects.

Gateways

A gateway performs routing functions. The term default gateway is used to identify the router that connects a LAN to an internet. A gateway can do more than a router; it also performs protocol conversions from one network to another.