Networking Equipment Terminology

Networking Equipment Terminology

Up to this point you have had only an introduction to the theory of the first two OSI layers. Now we'll cover the hardware used to implement them.

Network Interface Cards

Your network interface card is also frequently called a NIC. Currently, the most common types of NIC used in the home and office are Ethernet and wireless Ethernet cards.

The Meaning of the NIC Link Light

The link light signifies that the NIC card has successfully detected a device on the other end of the cable. This indicates that you are using the correct type of cable and that the duplex has been negotiated correctly between the devices at both ends.

Duplex Explained

Full duplex data paths have the capability of allowing the simultaneous sending and receiving of data. Half duplex data paths can transmit in both directions too, but in only one direction at a time.

Full duplex uses separate pairs of wires for transmitting and receiving data so that incoming data flows don't interfere with outgoing data flows.

Half duplex uses the same pairs of wires for transmitting and receiving data. Devices that want to transmit information have to wait their turn until the coast is clear, at which point they send the data. Error-detection and data-retransmission mechanisms ensure that the data reaches the destination correctly and are specifically designed to remedy data corruption caused when multiple devices start transmitting at the same time.

A good analogy for full duplex communications is the telephone, in which both parties can speak at the same time. Half duplex, on the other hand, is more like a walkie-talkie in which both parties have to wait until the other is finished before they can speak.

Data transfer speeds will be low and error levels will be high if you have a device at one end of a cable set to full duplex and a device at the other end of the cable set to half duplex.

Most modern network cards can autonegotiate duplex with the device on the other end of the wire. It is for this reason that duplex settings aren't usually a problem for Linux servers.

The MAC Address

The media access control (MAC) address can be equated to the serial number of the NIC. Every IP packet is sent out of your NIC wrapped inside an Ethernet frame that uses MAC addresses to direct traffic on your locally attached network.

MAC addresses therefore have significance only on the locally attached network. As the packet hops across the Internet, its source/destination IP address stays the same, but the MAC addresses are reassigned by each router on the way using a process called ARP.

How ARP Maps the MAC Address to Your IP Address

The Address Resolution Protocol (ARP) is used to map MAC addresses to network IP addresses. When a server needs to communicate with another server it takes the following steps:

Chapter 3, which covers Linux network topics, shows how to see your ARP table and the MAC addresses of your server's NICs.

The Two Broad Types of Networking Equipment

There are two main types of networking equipment: Data Communications Equipment (DCE), which is intended to act as the primary communications path, and Data Terminal Equipment (DTE), which acts as the source or destination of the transmitted data.

Data Terminal Equipment

DTE devices were originally computer terminals located at remote offices or departments that were directly connected modems. The terminals would have no computing power and only functioned as a screen/keyboard combination for data processing.

Nowadays most PCs have their COM and Ethernet ports configured as if they were going to be connected to a modem or other type of purely networking-oriented equipment.

Data Communications Equipment

DCE is also known as Data Circuit-Terminating Equipment and refers to such equipment as modems and other devices designed primarily to provide network access.

Using Straight-Through/Crossover Cables to Connect DTEs and DCEs

When a DCE is connected to a DTE, you need a straight-through cable. DCEs connected to DCEs or DTEs connected to DTEs require crossover cables. This terminology is generally used with Ethernet cables.

The terminology can be different for cables used to connect serial ports together. When connecting a PC's COM port (DTE) to a modem (DCE) the straight-through cable is frequently called a modem cable. When connecting two PCs (DTE) together via their COM ports, the crossover cable is often referred to as a null modem cable.

Some manufacturers configure the Ethernet ports of their networking equipment to be either of the DTE or the DCE type, and other manufacturers have designed their equipment to flip automatically between the two types until it gets a good link. As you can see, confusion can arise when selecting a cable. If you fail to get a link light when connecting your Ethernet devices together, try using the other type of cable.

A straight-through Ethernet cable is easy to identify. Hold the connectors side by side, pointing in the same direction with the clips facing away from you. The color of the wire in position #1 on connector #1 should be the same as that of position #1 on connector #2. The same would go for positions #2 through #8that is, the same color for corresponding wires on each end. A crossover cable has them mixed up. Figure provides some good rules of thumb.

Connectivity Using Hubs

A hub is a device into which you can connect all devices on a network so that they can talk together. Hubs physically cross-connect all their ports with one another, which causes all traffic sent from a server to the hub to be blurted out to all other servers connected to that hub whether they are the intended recipient or not.

Hubs have no, or very little, electronics inside and therefore do not regulate traffic. It is possible for multiple servers to speak at once, with all of them receiving garbled messages. When this happens the servers try again, after a random time interval, until the message gets through correctly.

It is for these reasons that Ethernet devices that plug into hubs should be set to half duplex.

Using Switches as a Faster Alternative to Hubs

A switch is also a device into which you can connect all devices on a home network so that they can talk together. Unlike a hub, traffic sent from Server A to Server B will be received only by Server B. The only exception is broadcast traffic, which is blurted out to all the servers simultaneously.

Switches regulate traffic, thereby eliminating the possibility of message garbling and providing a more efficient traffic flow.

Devices that plug into switches should be set to full duplex to take full advantage of the dedicated bandwidth coming from each switch port.

Local Area Networks

A local area network (LAN) is a grouping of ports on a hub, switch, or tied to a wireless access point (WAP) that can communicate only with each other.

It is possible to connect multiple switches and/or hubs in a chain formation to create a LAN with more ports. This is often called daisy chaining.

Switches and hubs provide no access control between servers connected to the same LAN. This is why network administrators group trusted servers having similar roles on the same LAN.

Servers use their IP address and subnet mask and the IP address of the remote server to determine whether they are both on the same network. If not, they attempt to communicate with each other via routers that interconnect their LANs. Routers are also capable of filtering traffic passing between the two LANs, therefore providing additional security.

Larger, more expensive switches can be configured to assign only certain ports to prespecified virtual LANs (VLANs) chosen by the network administrator. In this case, the switch houses ports on multiple LANs. A router still needs to be connected to each VLAN for internetwork communication.

How Routers Interconnect LANs

As stated before, switches and hubs usually have only servers connected to them that have been configured as being part of the same network. By connecting its NIC cards to multiple LANs, a correctly configured router is capable of relaying traffic between networks.

Routers can also be configured to deny communication between specific servers on different networks. They can also filter traffic based on the TCP port section of each packet. For example, it is possible to deny communication between two servers on different networks that intend to communicate on TCP port 80 and allow all other traffic between them. Routers therefore direct and regulate traffic between separate networks, much like a traffic policeman.

If you intend to route between networks, you must reserve an IP address for a router for each network and make sure that the router is directly connected to the LAN associated with that network.

In home networks, routers most frequently provide connectivity to the Internet using network address translation or NAT. In other words, routers act as gateways to the wider world and it won't be surprising to learn that routers are frequently referred to as "gateways."

How Simple Routing Works

In the broader networking sense, a "route" refers to the path data takes to traverse from its source to its destination. Each router along the way may also be referred to as a hop.

Usually when we speak about a route on a Linux box, we are referring to the IP address of the first hop needed to reach the desired destination network. It is assumed that this first hop will know how to automatically relay the packet.

Routers are designed to exchange routing information dynamically and can therefore intelligently redirect traffic to bypass failed network links. Home Linux boxes frequently don't run a dynamic routing protocol and therefore rely on static routes issued by the system administrator at the command line or in configuration files to determine the next hop to all desired networks.

Chapter 3, which covers Linux network topics, shows how to add static routes to your Linux box and also how you can convert it into a simple router.

Default Gateways, the Routers of Last Resort

A default gateway is the router that is used when no alternative devices can be found to relay the traffic. They are often called routers of last resort.

Say for example you have two routers, R1 and R2. R1 is connected to both your SOHO home network and the Internet. R2 is connected to SOHO home network and is capable of relaying data to other corporate networks with addresses starting with 10.X.X.X via another NIC card.

You could put a route on your SOHO servers that states:

• Go to network 10.0.0.0 255.0.0.0 via router R2.

• Go to everything else via router R1. R1 therefore would be considered your default gateway.

For most home networks, your default gateway would be the router/ firewall connected to the Internet.

Chapter 3, which covers Linux network topics, shows how to configure the default gateway on your Linux box.

Firewalls Help Provide a Secure Routing Environment

Firewalls can be viewed as routers with more enhanced abilities to restrict traffic, not just by port and IP address as routers do. Specifically, firewalls can detect malicious attempts to subvert the TCP/IP protocol. A short list of capabilities includes the following:

• Throttling traffic to a server when too many unfulfilled connections are made to it

• Restricting traffic being sent to obviously bogus IP addresses

• Providing network address translation or NAT

Routers are designed to make packets flow as quickly as possible with the minimum amount of inspection. Firewalls are used as close to the source or target of data communication as possible to try to ensure that the data hasn't been subverted.

Firewalls can often create an encrypted data path between two private networks across the Internet providing secure communication with a greatly reduced chance of eavesdropping. These VPNs are frequently used to connect branch offices to the corporate headquarters and also to allow sales representatives to get access to sensitive pricing information when traveling from town to town.