Juniper® JNCIA Exam Cram Notes : Layer2 Addressing And Address Resolution

1. Networking Fundamentals

1.5 Layer2 addressing and address resolution

The Internet is a global, public network with IP subnets connected by routers and exchanging packets. Both Ethernet and IP use globally unique network addresses that can be used as the basis for a truly global network. Ethernet MAC addresses come from the IEEE and IP subnet addresses come from various Internet authorities. (IP also employs a naming convention absent in Ethernet, but we'll ignore that in this discussion.) The key differences in how these addresses are assigned make all the difference when it comes to the basic functions of a bridge as opposed to a router.

All devices on LANs that are attached to the Internet have both MAC layer and IP addresses. Frames and packets contain both source and destination addresses in their headers.

In general:

  • MAC addresses are 48 bits long. The first 24 bits are assigned by the IEEE and form the organizationally unique identifier (OUI) of the manufacturer or vendor requesting the address. The last 24 bits form the serial number of the LAN interface cards and their uniqueness must be enforced by the company (some companies reuse numbers of bad or returned cards while others do not).
  • IPv4 addresses are 32 bits long. A variable number of the beginning bits are assigned by an Internet authority and represent a subnet located somewhere in the world. The remaining bits are assigned locally and, when joined to the network portion of the address, uniquely identify some host on a particular network.
  • IPv6 addresses are 128 bits long.
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Virtual Local Area Network (VLAN) creates a separate broadcast domain in layer 2 network. VLANs can also help create multiple layer 3 networks on a single physical infrastructure.

Address Resolution: ARP is the Address Resolution Protocol, which maintains a table named as ARP table. Sending IP packets on a multi access network requires mapping from an IP address to a media access control (MAC) address (the physical or hardware address).

In an Ethernet environment, ARP is used to map a MAC address to an IP address. ARP dynamically binds the IP address (the logical address) to the correct MAC address. Before IP unicast packets can be sent, ARP discovers the MAC address used by the Ethernet interface where the IP address is configured.

Hosts that use ARP maintain a cache of discovered Internet-to-Ethernet address mappings to minimize the number of ARP broadcast messages. To keep the cache from growing too large, an entry is removed if it is not used within a certain period of time. Before sending a packet, the host searches its cache for Internet-to-Ethernet address mapping. If the mapping is not found, the host sends an ARP request.

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