OFFICIAL cert guide CCENT Study guide questions (MAC Addresses)

gbdavidx

So I'm reading through his book, very detailed and so far easy to understand, however I noticed he doesn't explain WHY we need certain things just like they're there

For example, why do we need a MAC Address when using DNS? Shouldn't we just need IP address, or does having a MAC address help verify it is who it says it is?

pamccabe

Well that DNS request wouldn't get very far if the switch didn't have a MAC address in the frame.

gbdavidx

still confused

oli356

How would your DNS request get to the DNS server without using a MAC?

linuxlover

If you want to get to the DNS server from your PC you need to go through a router. You don't connect directly to a DNS server, your connection goes through many routers before reaching destination. Each of these routers have IN and OUT doors where connection enters and exists and each of these doors have their own physical addresses (MAC).

So, when you send a packet from your PC to the DNS server, it first enters your home router through the entrance (which has its own MAC address) and then exits through another door (which also have their own address). Then it travels to another router in the network again through the entrance (again its own MAC address) and exits through another door (again its own MAC address) so that it could do that a hundred times before it finally reached the DNS server.

The DNS server's IP address (destination IP address) always stays the same when packets travel as well as your PC's IP address (source IP address), but physical addresses or MAC addresses constantly change every time the packet goes through the router in the network, because each router has its own address.

If your PC is connected to the DNS server with a serial cable that would mean a direct connection, but that isn't the case. Between you and the DNS server lie hundreds of routers and when you send a packet to the DNS server, it goes from one router to another again and again until it reaches destination. So the IP address only tells what network the server resides in, but since the packet can't jump from NYC (network) over to LA (network) over the sky it needs to go through the middle-men (routers) to reach that network and each middle-man (router) has it's own address (MAC).

I hope that makes it clear, at least this is how I understand it. I'm not Cisco certified so I might be wrong.

pamccabe

linuxlover is correct, the PC will do Boolean logic on its own IP and subnet mask to find what network it is on. Then it will do Boolean logic on the destination DNS server's IP address. Your PC would see they sit on different networks and then forward the request to the default gateway, with the gateway's (router) MAC address as destination in the Frame.

pamccabe

gbdavidx wrote: »

still confused

I just meant that a switch is a layer 2 device that makes forwarding decisions based on MAC addresses. If a PC sent a packet without a MAC, and only an IP address, it wouldn't know what to do with it. Even our multi-function home routers/wireless access points perform switching functions.

d6bmg

gbdavidx wrote: »

still confused

When you send the first request to DNS server, router is going to need DNS server's MAC address which it will get via ARP.