Why IP AND MAC Address?

Dingdongbubble

I am studying for my Network+. I ahve a small question:

Why do they have two addresses? Why do they have both the IP and the MAC Address? Why not just the MAc Address?

Mishra

MAC addresses do not have the ability to be subnetted. MAC addresses also are just used to identify the actual NIC. Remember, every NIC has a unique MAC address. But if I were asking this question, it would help me a lot to know something that uses both technologies. So hopefully to give an example where both are used will help.

NAT - Network Address Translation:

When you have a router at home, it runs network address translation to be able to use 1 IP address for multiple computers in a LAN. When computers want to pass traffic to the internet, it will pass through the router. What the router does is strips the internal IP address (192.168.0.100 or whatever) and will put the public IP address of the router in its place (www.whatismyip.com). But in layer 2 it will continue to store the MAC address of the computer while passing traffic to the internet. When the packet comes back to the router by using the public IP instead of the internal IP, it can take the MAC address that was still stored in the packet information and look at its ARP table. The ARP table will tell the router its internal IP and it can then send that information to the correct computer on the LAN.

Dingdongbubble

hmmm so instead of all this why dont people jsut communicate on the internet using their MAc addresses? They would be represented as individual hosts on the internet instead of all this translation and routing business.

I am new so maybe my questions sound dumb.

When the packet already contains the MAc address tehn why does it ahve to ahve an IP address as well. Whats the point of having a logical IP address?

supertechCETma

MAC - physical address - fixed
IP - logical address - assignable

There are not enough MAC addresses in the world. IP allows the assignment of private class addresses, routing, subnetting, VPN, etc.

sprkymrk

Dingdongbubble wrote:

hmmm so instead of all this why dont people jsut communicate on the internet using their MAc addresses? They would be represented as individual hosts on the internet instead of all this translation and routing business.

I am new so maybe my questions sound dumb.

When the packet already contains the MAc address tehn why does it ahve to ahve an IP address as well. Whats the point of having a logical IP address?

MAC addresses can't be grouped logically. Think of it as everyone having a phone number containing 50 numbers. You don't use an area code, because everyone has a unique number, but you also don't know if they live in Chicago, Canada or China. Think of how the phone company would have to route calls when this is the case. The equipment they use would be much slower because every device would have to contain a database of every phone number in the world.

Mishra

Dingdongbubble wrote:

hmmm so instead of all this why dont people jsut communicate on the internet using their MAc addresses? They would be represented as individual hosts on the internet instead of all this translation and routing business.

I am new so maybe my questions sound dumb.

When the packet already contains the MAc address tehn why does it ahve to ahve an IP address as well. Whats the point of having a logical IP address?

"MAC addresses do not have the ability to be subnetted." Would be one answer to your question. I'm sure there would be some serious security concerns as well.

Now if your addition question is "well why don't they add subnets to MAC addresses" well you have gone beyond just understanding the technology to trying to redesign it and since I don't feel like digging up RFC articles nor was around when they were creating the tech then I couldn't answer that question.

Dingdongbubble

^so thats the only reason for having IP addresses?

I ahve another question which isnt exactly related but oh well:
In a bus topology, what gives the hosts an IP address? do they have a DHCP server? Where does a DHCP server exist?

sprkymrk

supertechCETma wrote:

There are not enough MAC addresses in the world. IP allows the assignment of private class addresses, routing, subnetting, VPN, etc.

MAC addresses are supposed to be unique, so in theory there are enough. The first half is the vendor number, the second half is specific to the individual device. Remember, mac addresses are based on hex, so there are about 16777215 available addresses for vendors, and 16777215 addresses per vendor, and most vendors are given multiple ranges since there are not 16 million vendors making networking devices.

In short: 16777215 vendors that can each have 16777215 addresses.

sprkymrk

Dingdongbubble wrote:

^so thats the only reason for having IP addresses?

Read my example about phone numbers and see if it makes sense.

Mishra

Dingdongbubble wrote:

^so thats the only reason for having IP addresses?

I ahve another question which isnt exactly related but oh well:
In a bus topology, what gives the hosts an IP address? do they have a DHCP server? Where does a DHCP server exist?

I'm for asking multiple questions in one thread.

TCP/IP is still used in a bus topology so yes your standard methods apply. They have DHCP servers. The difference between bus and star is how data is transmitted.

EDIT: But I will add that most people used static addressing when bus topology was around.

Dingdongbubble

Right thanks for the replies.

I studying ARP so I hope you cna bear with me.

When using ARP, I have the IP address but not the MAC address. So if I have MAC address but not the IP address what protocol will I use to get the IP address?

If I use this on the internet, will the ARP request be sent to every single device on the internet and will all switches forward this request too? If this Is done then isnt it possibe to bog down a network by sending PING or ARP requests?

Thanks

Mishra

Dingdongbubble wrote:

Right thanks for the replies.

I studying ARP so I hope you cna bear with me.

When using ARP, I have the IP address but not the MAC address. So if I have MAC address but not the IP address what protocol will I use to get the IP address?

If I use this on the internet, will the ARP request be sent to every single device on the internet and will all switches forward this request too? If this Is done then isnt it possibe to bog down a network by sending PING or ARP requests?

Thanks

It is called RARP or Reverse address resolution protocol.

There is nothing you can do while using the internet that will send information to every single device on the internet. A BROADCAST (caps for it being a key term) is what is used by DHCP and ARP and other protocols and they are stopped by routers (broadcast domains). So the first router the packet will see is the router which will stop that broadcast.

EdTheLad

Dingdongbubble wrote:

hmmm so instead of all this why dont people jsut communicate on the internet using their MAc addresses? They would be represented as individual hosts on the internet instead of all this translation and routing business.

I am new so maybe my questions sound dumb.

When the packet already contains the MAc address tehn why does it ahve to ahve an IP address as well. Whats the point of having a logical IP address?

Lets look at the scenario with only mac addresses.Mac addresses are layer 2, before sending a layer 2 frame my pc needs to add a source and destination mac-address.When this frame is sent,every other device on the wire must examine this frame to see if the destination mac is addressed for it.This means the more devices i add to the local wire the more flooding, soon your pc nic will spend all its time looking at unwanted frames and have no time to receive frames destined for it.Now you decide to fix the problem with a switch,this means every device is on its own wire a the switch will map mac addresses to ports so the flooding problem is over.
Now as time goes by we add more and more devices and soon the switch is fully populated,now we add another switch and another and soon we have too many to count.The mac-address table on the switches get bigger and bigger and the length of time to do a table look up gets longer and longer, soon the end applications will timeout and send the frames again.So we have a hugh mess.
How do we fix this problem? by using a hieracrchial addressing structure, ip is the solution, layer 2 addresses i.e. the mac address is used to address a physical port, an ip address is used to address a network.So by adding routers i can segment my layer 2 network into multiple layer 2 networks.The mac address is now used to get to my router, the router will look at the frame, see the mac address is destined for its local port, pass the data to its upper ip layer, the router will look at the ip address fields, check its routing table for the destination network, send the frame out the correct port as specified in the routing table.The frame will have the source mac address of the routers exit interface and the destination mac address of the next-hop router, or if this is the destination network, it will have the mac address of the destination device.How these mac-addresses/ip address combinations are learned is down to a protocol called arp.
Dont know if this is too much for you,anyway i tried!

Dingdongbubble

^oh ok. Thanks for pointing the word broadcast out. Havent hackers figured out a way of overriding routers killing broadcasts?

Thanks for the reply edtheland. I will read it with more concentration in a lil while.

EdTheLad

Dingdongbubble wrote:

^oh ok. Thanks for pointing the word broadcast out. Havent hackers figured out a way of overriding routers killing broadcasts?

You're probably referring to denial of service attack.Newer routers have a forwarding plane and a control plane.The control plane is accessed by the cpu, the forwarding plane uses fast switching in hardware.If you apply a control plane policy-map, i.e. rate limit the control plane you shouldn't have a problem with broadcasts killing a router.

Dingdongbubble

^I practically understood nothing. I am just a newbie. Soon one day I will hopefully.

KGhaleon

Load up a protocol analyzer on your computer. MAC and IP work on completely different layers. Routers don't route Mac addresses, and switches don't route IP addresses.

hetty

Layer 7 - Application - DNS, FTP, Telnet, HTTP
Layer 6 - Presentation - File types & Data types
Layer 5 - Session - Session tracking
Layer 4 - Transport - Transport of IP via TCP (connections) or UDP (connectionless). Even through a VPN tunnel using PPTP, L2TP or IPSec.
Layer 3 - Network - IP addresses assigned manually or by DHCP to MAC addresses (specific machines). If it can't find the MAC address of an IP address it processes an ARP request to find out what the MAC address it is.
Layer 2 - Data Link - Ethernet & Wireless frames (envelopes of data) for specific machines based on their globally unique MAC address.
Layer 1 - Physical - The Physical 0s & 1s of electrical signals on the copper wire.

As said previously you cannot access a MAC address over the internet (routed) because the internet uses TCP/IP, which works at Layer 3, like broadband routers and modems. And switches usually don't work using IP address and can't be accessed over the internet so they usually just work with MAC addresses. So you kind of need both hand in hand for a PC on the network to access the internet. Each layer passes information to the next, up or down depending on which direction the communication is taking place in.

http://en.wikipedia.org/wiki/OSI_model#Examples

Plantwiz

supertechCETma wrote:

There are not enough MAC addresses in the world. IP allows the assignment of private class addresses, routing, subnetting, VPN, etc.

YES, and there are some MAC addresses that have come back into use...so there could be more then 1 of an address.



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Never mind...

Looks like Spymrk already answered this (and used the same quote AND did a better job