Hi Oscar,
Q1: "While an IP packet is being packaged to be delivered at the Ethernet
frame stage, how is the destination MAC address determined?"
A1: The Address Resolution Protocol (ARP) is used to determine the
destination MAC address when only the destination IP is known. The source
host will broadcast an ARP request to all hosts on the local network asking
the owner of the destination IP address to respond. Only the host that
owns the destination IP will respond (using a unicast packet) with an ARP
reply saying "Here's my MAC address...". All other hosts will ignore the
ARP request since it does not pertain to them.
Q2: "Is the destination MAC address going to be the MAC address of the
local gateway or the remote host?"
A2: It depends. If the destination host is on the local network (i.e.,
source & destination are connected to the same segment) then the
destination MAC will the MAC address of the remote host. However, if the
destination host is on a remote network then the destination MAC will be
the address of the local gateway (router). The reason for this is that the
ARP request (which is broadcast) will not be forwarded by the
router. Therefore, the remote host will never have a chance to reply to
the ARP request since it will never see it. In cases such as these, the
router will respond to an ARP request on behalf of a remote host - this is
known as Proxy ARP.
Q3: "Is the MAC changed by the network devices (routers) along the way
until it has been delivered to the destination Ethernet IP address?"
A3: Yes, the destination MAC addresses will change hop to hop (router to
router) as the packet travels across the network. On the other hand, the
destination IP address will remain the same until it reaches its destination.
Q4: "Which half is the vendor specific portion?"
A4: The vendor specific portion of the MAC address (also known as the OUI -
Organizationally Unique Identifier) is the first 24 bits (3 bytes) of the
MAC. In your example, this would be 01 23 45.
Q5: "Where would the multicast bit and locally administered MAC address bit
be located?"
A5: The multicast bit is the low-order bit of the first octet of an
ethernet address. This bit should be set to 1 for multicast mode. For
example, given the MAC address 08 01 02 03 04 05, the multicast address
would be 09 01 02 03 04 05 (last bit in the first byte changed from 0 to
1). As far as the locally administered bit is concerned, that should be
bit number 7 (out of 48) of the MAC. Again, 1 means local, 0 means global
or IEEE administered.
HTH,
Marcus
At 10:46 PM 08/08/2000 +0000, Oscar Rau wrote:
>While an IP packet is being packaged to be delivered at the Ethernet frame
>stage,
>how is the destination MAC address determined? Is the destination MAC
>address going
>to be MAC address of the local gateway or the remote host?
>
>Is the MAC changed by the network devices (routers) along the way until it
>has been delivered to
>the destination Ethernet IP address?
>
>If a MAC address is,
>
> 01 23 45 67 89 11
>
>Which half is the vendor specific portion? Where would the multicast bit
>and locally
>administered MAC address bit be located?
>
>Thank you in advance.
>--
>
>Oscar Rau
>[EMAIL PROTECTED]
>
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=====================================
Marcus Walton
Lucent Technologies, Inc.
NetworkCare Professional Services Division
"The Knowledge Behind the Network"
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