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A crossover cable connects two devices of the same type, for example DTE-DTE or DCE-DCE, usually connected asymmetrically (DTE-DCE), by a modified cable called a crosslink. Such distinction of devices was introduced by IBM.

The crossing wires in a cable or in a connector adaptor allows:

connecting two devices directly, output of one to input of the other,
letting two terminal (DTE) devices communicate without an interconnecting hub knot, i.e. PCs,
linking two or more hubs, switches or routers (DCE) together, possibly to work as one wider device.
[edit] Examples
a Null modem of RS-232
Ethernet crossover cable
Rollover cable
A loopback is a type of degraded "one side crosslinked connection" connecting a port to itself, usually for test purposes.
Use crossover cables for the following connections: Switch to switch, Switch to hub, Hub to hub, Router to router, PC to PC, Router to PC

[edit] Other technologies
Some connection standards use different balanced pairs to transmit data, so crossover cables for them have different configurations to swap the transmit and receive pairs:

Twisted pair Token ring uses T568B pairs 1 and 3 (the same as T568A pairs 1 and 2), so a crossover cable to connect two Token Ring interfaces must swap these pairs, connecting pins 4, 5, 3, and 6 to 3, 6, 4, and 5 respectively.
A T1 cable uses T568B pairs 1 and 2, so to connect two T1 CSU/DSU devices back-to-back requires a crossover cable that swaps these pairs. Specifically, pins 1, 2, 4, and 5 are connected to 4, 5, 1, and 2 respectively.
A 56K DDS cable uses T568B pairs 02 and 04, so a crossover cable for these devices swaps those pairs (pins 01, 02, 07, and 08 are connected to 07, 08, 01, and 02 respectively).
A null modem cable.
An Ethernet crossover cable

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Although no maximum length for an HDMI cable is specified, signal attenuation—dependent on the cable's construction quality and conducting materials—limits usable lengths in practice.[57] HDMI 1.3 defines two cable categories: Category 1-certified cables, which have been tested at 74.5 MHz (which would include resolutions such as 720p60 and 1080i60), and Category 2-certified cables, which have been tested at 340 MHz (which would include resolutions such as 1080p60 and 2160p30).[54][58][59] Category 1 HDMI cables are to be marketed as "Standard" and Category 2 HDMI cables as "High Speed".[1] This labeling guideline for HDMI cables went into effect on October 17, 2008.[60][61] Category 1 and 2 cables can either meet the required parameter specifications for interpair skew, far-end crosstalk, attenuation, and differential impedance, or they can meet the required nonequalized/equalized eye diagram requirements.[58] A cable of about 5 meters (16 ft.) can be manufactured to Category 1 specifications easily and inexpensively by using 28 AWG (0.081 mm²) conductors.[57] With better quality construction and materials, including 24 AWG (0.205 mm²) conductors, an HDMI cable can reach lengths of up to 15 meters (49 ft.).[57] Many HDMI cables under 5 meters of length that were made before the HDMI 1.3 specification can work as Category 2 cables, but only Category 2-tested cables are guaranteed to work

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Straight Cable

You usually use straight cable to connect different type of devices. This type of cable will be used most of the time and can be used to:

1) Connect a computer to a switch/hub's normal port.
2) Connect a computer to a cable/DSL modem's LAN port.
3) Connect a router's WAN port to a cable/DSL modem's LAN port.
4) Connect a router's LAN port to a switch/hub's uplink port. (normally used for expanding network)
5) Connect 2 switches/hubs with one of the switch/hub using an uplink port and the other one using normal port.

If you need to check how straight cable looks like, it's easy. Both side (side A and side B) of cable have wire arrangement with same color. Check out different types of straight cable that are available in the market here.

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An Internet Protocol (IP) address is a numerical label that is assigned to devices participating in a computer network, that uses the Internet Protocol for communication between its nodes.[1] An IP address serves two principal functions: host or network interface identification and location addressing. Its role has been characterized as follows: "A name indicates what we seek. An address indicates where it is. A route indicates how to get there."[2]

The designers of TCP/IP defined an IP address as a 32-bit number[1] and this system, known as Internet Protocol Version 4 or IPv4, is still in use today. However, due to the enormous growth of the Internet and the predicted depletion of available addresses, a new addressing system (IPv6), using 128 bits for the address, was developed in 1995[3] and standardized by RFC 2460 in 1998.[4] Although IP addresses are stored as binary numbers, they are usually displayed in human-readable notations, such as 208.77.188.166 (for IPv4), and 2001:db8:0:1234:0:567:1:1 (for IPv6).

The Internet Protocol is used to route data packets between networks; IP addresses specify the locations of the source and destination nodes in the topology of the routing system. For this purpose, some of the bits in an IP address are used to designate a subnetwork. The number of these bits is indicated in CIDR notation, appended to the IP address; e.g., 208.77.188.166/24.

As the development of private networks raised the threat of IPv4 address exhaustion, RFC 1918 set aside a group of private address spaces that may be used by anyone on private networks. They are often used with network address translators to connect to the global public Internet.

The Internet Assigned Numbers Authority (IANA), which manages the IP address space allocations globally, cooperates with five Regional Internet Registries (RIRs) to allocate IP address blocks to Local Internet Registries (Internet service providers) and other entities