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Packets are only passed to the network segment they are
destined for. They work similar to bridges and switches
in that they filter out unnecessary network traffic and
remove it from network segments. Routers generally work
at the protocol level.
Routers were devised in order to separate networks logically. For instance, a TCP/IP router can segment the network based on groups of TCP/IP addresses. Filtering at this level (on TCP/IP addresses, also known as level 3 switching) will take longer than that of a bridge or switch which only looks at the MAC layer.
Most routers can also perform bridging functions. A major feature of routers, because they can filter packets at a protocol level, is to act as a firewall. This is essentially a barrier, which prevents unwanted packets either entering or leaving designated areas of the network.
Typically, an organization which connects to the Internet will install a router as the main gateway link between their network and the outside world. By configuring the router with access lists (which define what protocols and what hosts have access) this enforces security by restricted (or allowing) access to either internal or external hosts.
For example, an internal WWW server can be allowed IP access from external networks, but other company servers which contain sensitive data can be protected, so that external hosts outside the company are prevented access (you could even deny internal workstations access if required).
A router works at the Network Layer or higher, by looking at information embedded within the data field, like a TCP/IP address, then forwards the frame to the appropriate segment upon which the destination computer resides.
Summary of Router features
Nowadays, with the advent of 10BaseT, hub concentrators are being very popular. These are very sophisticated and offer significant features which make them radically different from the older hubs which were available during the 1980's.
These 10BaseT hubs provide each client with exclusive access to the full bandwidth, unlike bus networks where the bandwidth is shared. Each workstation plugs into a separate port, which runs at 10Mbps and is for the exclusive use of that workstation, thus there is no contention to worry about like in Ethernet.
These 10BaseT hubs also include buffering of packets and filtering, so that unwanted packets (or packets which contain errors) are discarded. SNMP management is also a common feature.
In standard Ethernet, all stations are connected to the same network segment in bus configuration. Traffic on the bus is controlled using the CSMA (Carrier Sense Multiple Access) protocol, and all stations share the available bandwidth.
10BaseT Hubs dedicate the entire bandwidth to each port (workstation). The workstations attach to the hub using UTP. The hub provides a number of ports, which are logically combined using a single backplane, which often runs at a much higher data rate than that of the ports.
Ports can also be buffered, to allow packets to be held in case the hub or port is busy. And, because each workstation has it's own port, it does not contend with other workstations for access, having the entire bandwidth available for it's exclusive use.
The ports on a hub all appear as one Ethernet segment. In addition, hubs can be stacked or cascaded (using master/slave configurations) together, to add more ports per segment. As hubs do not count as repeaters, this is a better option for adding more workstations than the use of a repeater.
Hub options also include an SNMP (Simple Network Management Protocol) agent. This allows the use of network management software to remotely administer and configure the hub. Detailed statistics related to port usage and bandwidth are often available, allowing informed decisions to be made concerning the state of the network.
In summary, the advantages for these newer 10BaseT hubs are,