CTI Products HIB-IP IP Interface - Manual (Page 30)

CTI Products, Inc. HIB-IP & HIB-IP 8000 Hardware Reference Manual
Appendix G. IP Addressing 30
MAC Addresses
Whereas IP uses Logical Addresses to identify a host (node), other protocols use Hardware Addresses called
Media Access Control addresses, or MAC addresses. MAC addresses are set at the factory at time of
manufacture and cannot be changed. (IP addresses can be changed at any time.) A MAC address consists of
six octets, in hexadecimal notation, separated by colons. An example would be:
00:10:EE:00:02:34.
The first three octets in a MAC address identify the manufacturer. In the above example, 00:10:EE identifies
CTI Products as the manufacturer of this device. The last three octets are sequentially assigned by the
manufacturer to form a type of serial number. In this way, no two devices have the same MAC address.
Address Resolution Protocol (ARP)
Since IP uses Logical Addresses at the OSI Network Layer (Layer 3) and Ethernet uses MAC addresses at the
OSI Data Link Layer (Layer 2), Address Resolution Protocol is used whenever IP is used over the Ethernet.
ARP is needed to convert an IP address to a MAC address. The client then stores this resolved address for a
period of time in its ARP cache. An ARP cache is a lookup table, typically in a router, that will store a quantity
of resolved addresses for devices that it must communicate with.
When a message is destined for an IP address whose MAC address has not yet been resolved, an ARP
REQUEST is sent from the local host as a broadcast message, asking for MAC identification. A remote host
with the IP address in question generates an ARP RESPONSE. This ARP RESPONSE contains the requested
MAC address. The local host receives the message, and places the IP Address and the matching MAC Address
in its ARP Cache. The original message is then sent using the MAC Address found previously. Any additional
messages to this remote IP address will be sent using the MAC address found in the local host’s ARP cache.
Ethernet 10BaseT Cables
When connecting two IP devices together, either a straight-through cable or a crossover cable may be required.
If interconnecting similar devices, then a crossover cable is needed. Examples would be interconnecting two
EXB-IP routers, or two IP routers, or two hubs, or two computers. Generally, if the two devices are both Layer
3 devices, then a crossover cable is required. Routers operate at Layer 3. Similarly, if the two devices are
Layer 1 or 2 devices, they would also require a crossover cable. Bridges, switches, and Network Interface
Cards (NIC) operate at Layer 2, and hubs, repeaters, and concentrators operate at Layer 1.
If interconnecting dissimilar devices, then a straight-through cable is needed. Dissimilar devices would involve
one Layer 3 device and a Layer 1 or Layer 2 device.
The following table summarizes device types in each layer.
Layer 3 Layer 2 Layer 1
IP Router Bridge Hub
EXB-IP Network Extender Switch Repeater
HIB-IP Unit NIC Concentrator