Juniper Networks 10.4

NOTE: Theopposite of a “globallyunique network address” is the “locally

significantconnectionidentifier ” which connectstwo endpoints on a network.

Forexample, MPLS labels such as 1000001 can repeatin a network, but a

publicIP address can appear on the Internet in only one place ata time

(otherwiseit is an error).

Alldevices on LANs that are attached to the Internethave both MAC layer and IP

addresses.Frames and packets contain both sourceand destination addresses in their

headers.In general:

•MACaddresses are 48 bits long. The first 24bits are assigned by the IEEE and form

theorganizationally unique identifier (OUI) of the manufactureror vendor requesting

theaddress. The last 24 bits form the serial number of the LAN interfacecards and

theiruniqueness must be enforced by the company (some companiesreuse numbers

ofbad or returned cards while others do not).

•IPv4addresses are 32 bits long. A variable number of the beginning bits are assigned

byan Internet authority and represent a subnet locatedsomewhere in the world. The

remainingbits are assigned locally and, when joined tothe network portion of the

address,uniquely identify some host on a particular network.

•IPv6addresses are 128 bits long. Although there are significantdifferences, for the

purposesof this discussion, it is enough to point out that there is also a network and

hostportion to an IPv6 address.

Notethat MAC addresses are mainly organizedby manufacturer and IP addresses are

organizedby network, which is locatedin a particular place. Therefore, the IP address

caneasily be used by routersfor a packet's overall direction (for example,“192.168.27.48

iswest of here”). However,the MAC addresses on a vendor's interface cards can end up

anywherein the world, and often do. Consider a Juniper Networksrouter as a simple

example.EveryEthernet LAN interface on the routerthat sends or receives packets places

theminside Ethernet frames with MAC addresses. All of theseinterfaces share the initial

24bits assigned toJuniper Networks. Two might differonly in one digit from one interface

toanother. Yetthe routers containing these MACinterfaces could be locatedon opposite

sidesof the world.

AnInternet backbone router only needs a table entry forevery network (not host) in the

world.Most other routers only havea portion of this full table, and a default route for

forwardingpackets with no entries in their table. In contrast,to perform the same role,

abridge would need one table entry for everyLAN interface, on host or bridge, in the

world.This is hard enough to do for Ethernetsthat span a metropolitan area, let alone

theentire world.

NOTE: Thereare other reasons thatEthernet would be hard-pressed to

becomea truly global network,including the fact that MAC addresses do not

oftenhave names associatedwith them while IP addresses do (for example,

192.168.27.48mightbe host48.accounting.juniper.net).This section addresses

onlythe address issues.

Junos10.4 MX Series Ethernet ServicesRouters Solutions Guide