UNIX for Dummies Questions & Answers

If a web search is done on this topic there are many links but no information included in the documents. So my questions are;

Why does a NIC card have it's own permanent ID. Why can't it share the host name? An ID is an ID.

Why does a computer need two ways to ID it.

If the network needs to have data carried with the packets in order for the network to navigate the packet it makes sense but no other reason does.

This must be a mostly misunderstood concept judging from the amount of "what is the differerence between a Mac and IP address" links so thanks for your patients and thanks in advance.

Because modern networks still use protocols that aren't based on IP, and/or are used before any IP configuration has happened.

As an everyday example, read the description of DHCP. All communication to get the IP configuration information is done through packets on the local network segment using non-routable MAC addresses. Only after that the device gets an IP that a router can transmit to other networks.

Another, even more common example, would be the Address Resolution Protocol (ARP). This uses the MAC address to broadcast the hosts IP to the network segment, so that switches and other hosts can update their network view, or query the network for who has a certain address. This is also used to detect duplicate IPs on the same segment.

When is just a Mac addressed used?

When is just a IP address used?

When are they both used in the same packet?

In some docs. it looks like the Mac address is only used from the network to the host but this is contradicted in other docs.

Why do the layers matter?

Thanks again! Edit: It looks like the term IP address should actually be IP Name because it is really not an address but an indirect way to aquire it.

The MAC address is used every time you send a packet, and there's no case where just the IP is used. Most traffic you'll see is either using TCP or UDP packets for transport. These are encapsulated in IP packets, which are in turn (on a regular ethernet or WiFI network) encapsulated in Ethernet frames. TCP and UDP know nothing about the network structure, and don't really care, because that's the job of IP. IP in turn doesn't know anything about the data it's transmitting, but only cares about getting it through the network. And Ethernet only cares about getting it through the wire to the next local point without colliding with another frame.

How do these parts play together? Lets say you want to establish a TCP connection between 2 hosts:
I'm assuming here that both hosts already know the IP of the other (either by a static entry or through DNS).

A data packet from Hoth to Dagobah can't be sent directly, since they're on different subnets, so it has to go through the gateway. But IP doesn't allow for multiple destination addresses. If the packet is sent with the gateway IP as destination, it won't be sent out on the other network. And we can't get the MAC address of the other host, since that information isn't routed to us. The solution is that the packet is sent with the IP address of Dagobah, but with the MAC address of the gateway. That way the routing gateway has to look at the packet (it was directed there), sees that the real destination is on the other network, and sends it there.