Networking Cables

Straight-through cables get their name from how they are made. Out of the 8 pins that exist on both ends of an Ethernet cable, each pin connects to the same pin on the opposite side. Review the diagram below for a visual example:

Notice how each wire corresponds to the same pin. This kind of wiring diagram is part of the 568A standard. The 568B standard achieves the same thing, but through different wiring. It is generally accepted to use the 568A standard as pictured, since it allows compatibility with certain telephone hardware- while 568B doesn’t.
Straight-through cables are primarily used for connecting unlike devices. A straight-through cable is typically used in the following situations:
Use a straight-through cable when:

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• 1. Connecting a router to a hub
• 2. Connecting a computer to a swtich
• 3. Connecting a LAN port to a switch, hub, or computer

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Note that some devices such as routers will have advanced circuitry, which enables them to use both crossover and straight-through cables. In general, however, straight-through cables will not connect a computer and router because they are not “unlike devices.”

The Purpose of Crossover Cables

Crossover cables are very similar to straight-through cables, except that they have pairs of wires that crisscross. This allows for two devices to communicate at the same time. Unlike straight-through cables, we use crossover cables to connect like devices. A visual example can be seen below:

Notice how all we did was switch the orange-white and green-white wires, and then the orange and green wires. This will enable like devices to communicate. Crossover cables are typically used in the following situations:
Use a crossover cable when:

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• 1. Connecting a computer to a router
• 2. Connecting a computer to a computer
• 3. Connecting a router to a router
• 4. Connecting a switch to a switch
• 5. Connecting a hub to a hub

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While the rule of thumb is to use crossover cables with like devices, some devices do not follow standards. Others provide support for both types of cables. However, there is still something that both crossover and straight-through cables can’t do.

The Purpose of Rollover Cables

Rollover cables, like other cabling types, got their name from how they are wired. Rollover cables essentially have one end of the cable wired exactly opposite from the other. This essentially “rolls over” the wires- but why would we need to do such a thing? Rollover cables, also called Yost cables, usually connect a device to a router or switch’s console port. This allows a programmer to make a connection to the router or switch, and program it as needed. A visual example can be seen below:

Notice that each wire is simply “rolled over.” These types of cables are generally not used very much, so are usually colored differently from other types of cables.

Network Devices

A router is an Open Systems Interconnect (OSI) model Layer 3 device. IP addressing and routing is performed at Layer 3. A router is used to connect networks that are assigned IP addresses in different subnets. A router delimits a broadcast domain.

A bridge is a Layer 2 device that connects network segments. A bridge is more efficient than a repeater because it can analyze traffic. However, it does not limit a broadcast domain.

A firewall is a security device that filters traffic. Basic firewalls filter incoming and outgoing traffic based on the TCP/IP port. A firewall is typically used between segments that have different security requirements. There are different types of firewalls that operate at different layers of the OSI model. For example, a packet filter firewall operates at the Layer 3 of the OSI model, but an application layer firewall operates at Layer 7.

A repeater also connects network segments. However, a repeater simply rebroadcasts the signal. It cannot be used to connect segments in different IP subnets. A repeater also does not serve as a broadcast domain delimiter. A repeater operates at Layer 1 of the OSI model.

IGMP

IGMP is used to control sending packets to broadcast groups. This enables one computer to send content to selected group of other computers in one broadcast, unlike a standard broadcast that is delivered to all computers. Also, routers pass IGMP messages, but block standard broadcast messages.