Hour 9

Network Hardware

What You’ll Learn in This Hour

Bridges

Hubs and switches

Routers

Network Address Translation

Connectivity Device

Traffic control

Connectivity

Hierarchical addressing

Signal regeneration

The primary uses of connectivity devices are as follow:

Bridges

A bridge is a connectivity device that filters and forwards packets by physical address. Bridges operate at the OSI Data Link layer

In recent years, bridges have become much less common as networks move to more versatile devices, such as switches.

A bridge listens to each segment of the network it is connected to and builds a table showing which physical address is on which segment.

When data is transmitted on one of the network segments, the bridge checks the destination address of the data and consults the MAC-Address table.

Bridges (continue)

If the destination address is on the segment from which the data was received, the bridge ignores the data.

If the destination address is on a different segment, the bridge forwards the data to the appropriate segment.

If the destination address isn't in the MAC-address table, the bridge forwards the data to all segments except the segment from which it received the transmission.

Bridges (continue)

Hub

As you'll recall from Hour 3, the classic Ethernet concept calls for all computers to share the transmission medium. Each transmission is heard by all network adapters.

An ethernet hub receives a transmission from one of its ports and echoes that transmission to all of its other ports (refer to Figure 9.2).

In other words, the network behaves as if all computers were connected using a single continuous line.

The hub does not filter or route any data. Instead, the hub just receives and retransmits signals.

Hub (continue)

One of the principal reasons for the rise of hub-based Ethernet is that in most cases a hub simplifies the task of wiring the network.

Each computer is connected to the hub through a single line.

A computer can easily be detached and reconnected.

In an office setting where computers are commonly grouped together in a small area, a single hub can serve a close group of computers and can be connected to other hubs in other parts of the network.

Hub (continue)

Switches A hub-based Ethernet network still faces the principal liability of Ethernet:

Performance degrades as traffic increases.

No computer can transmit unless the line is free. Furthermore, each network adapter must receive and process every frame placed on the Ethernet.

A smarter version of a hub, called a switch, was developed to address these problems with Ethernet.

In its most fundamental form, a switch looks very similar to the hub shown in Figure 9.2.

Each computer is attached to the switch through a single line. However, the switch is smarter about where it sends the data received through one of its ports.

Switches (continue)

Most switches associate each port with the physical address of the adapter connected to that port (see Figure 9.3).

When one of the computers attached to the port transmits a frame, the switch checks the destination address of the frame and sends the frame to the port associated with that destination address.

In other words, the switch sends the frame only to the adapter that is supposed to receive it.

Every adapter does not have to examine every frame transmitted on the network. The switch reduces superfluous transmissions and therefore improves the performance of the network.

Switches (continue)

Switches (continue)

Switches Type

Cut-through The switch starts forwarding the frame as soon as it obtains the destination address.

Store and forward The switch receives the entire frame before retransmitting. This method slows down the retransmission process, but it can sometimes improve overall performance because the switch filters out fragments and other invalid frames.

Routers A router is a device that filters traffic by logical address. Routers operate at the Internet

layer (OSI Network layer) using IP addressing information in the Internet layer header.

Routers are an essential part of any large TCP/IP network. Without routers the Internet could not function. In fact, the Internet never would have grown to what it is today without the development of network routers and TCP/IP routing protocols.

A large network such as the Internet contains many routers that provide redundant pathways from the source to the destination nodes. The routers must work independently, but the effect of the system must be that data is routed accurately and efficiently through the internetwork.

Routers are far more sophisticated than bridges. Routers replace Network Access layer header information as they pass data from one network to the next, so a router can connect dissimilar network types. Many routers also maintain detailed information describing the best path based on considerations of distance, bandwidth, and time.

What is Router? The best way to describe a router is to describe how it looks. In its

simplest form a router looks like a computer with two network adapters. The earlier routers were actually computers with two or more network adapters (called multihomed computers). Figure 9.5 shows a multihomed computer acting as a router.

Routers (in a real world) The router has more than two ports (adapters) and can therefore

interconnect more than two networks. The decision of where to forward the data then becomes more complicated, and the possibility for redundant paths increases.

The networks that the router interconnects are each interconnected with other networks. In other words, the router sees network addresses for networks to which it is not directly connected. The router must have a strategy for forwarding data addressed to networks to which it is not directly attached.

The network of routers provides redundant paths, and each router must have a way of deciding which path to use.

Complex Network and Routing

Introduction to Routing

The router receives data from one of its attached networks.

The router passes the data up the protocol stack to the Internet layer. In other words, the router discards the Network Access layer header information and reassembles (if necessary) the IP datagram.

The router checks the destination address in the IP header. If the destination is on the network from whence the data came, the router ignores the data. (The data presumably has already reached its destination because it was transmitted on the network of the destination computer.)

Introduction to Routing (cont)

If the data is destined for a different network, the router consults a routing table to determine where to forward the data.

After the router determines which of its adapters will receive the data, it passes the data down through the appropriate Network Access layer software for transmission through the adapter.

The Routing Process

Primary Types of Routing

Static routing Requires the network administrator to enter route information manually.

Dynamic routing Builds the routing table dynamically based on routing information obtained using routing protocols.

Routing Table Concept

A routing table essentially maps destination network IDs to the IP address of the next hop—the next stop the datagram makes on its path to the destination network.

Note that the routing table makes a distinction between networks directly connected to the router itself and networks connected indirectly through other routers.

The next hop can be either the destination network (if it is directly connected) or the next downstream router on the way to the destination network. The Router Port Interface in Figure 9.8 refers to the router port through which the router forwards the data.

Routing Table Concept

Network Address Translation (NAT) A network address translation device obscures all details of the local

network and hides the very existence of the local network. Figure 9.9 shows a network address translation device on the Internet.

The NAT device serves as a gateway for computers on the local network to access the Internet. Behind the NAT device, the local network can use any network address space.

The network does not have to use specially assigned Internet addresses because the local network is not even part of the Internet.

The NAT device instead acts as a proxy for the local network on the Internet.

NAT (continue)

When a local computer attempts to connect to an Internet resource, the NAT device makes the connection instead.

Any packets received from the Internet resource are translated into the address scheme of the local network and forwarded to the local computer that initiated the connection.

NAT (continue)