computer networks

The Answer To The Media Access Control Question
----------------------------
Over the past few weeks I have received quite a few e-mails about Ethernet cards
both wired and wireless
and more specifically
about Media Access Control (MAC) addresses. I think the main reason I’ve received so many questions about Ethernet cards and MAC addresses is people trying to secure their home wireless networks and their desire to use MAC address filtering. This type of filtering in wireless networks can be configured to allow or deny specific computers to use or attach to the wireless network
based on the MAC address.

My first thought was to write an article just about MAC addresses and wireless Ethernet. After thinking about it I decided to expand on this and go over some specific information about Ethernet cards and communication.

Different Ways Of Finding Your MAC Address And More
--------------------------------------------------
There are several ways of finding your Ethernet and communications protocol information. Many Ethernet card manufacturer’s have proprietary software that can reveal this information but they work differently depending on the manufacturer. So we will use the Windows 2000 and XP “ipconfig” utility since this is available in the majority of Windows Operating Systems.

First
go to “start” -> “run” and type “cmd” without the quotes. Then hit the enter key. At the command line type “ipconfig /all”
again without the quotes. Actually
just typing ipconfig without the /all will work but will only provide you with abbreviated information regarding your network cards. An example of what you might see by typing the “ipconfig /all” command is below with each item commented in green lettering:

Fault Tolerant And Highly Availability Computer Systems
----------------------------
There are several ways of finding your Ethernet and communications protocol information. Many Ethernet card manufacturer’s have proprietary software that can reveal this information but they work differently depending on the manufacturer. So we will use the Windows 2000 and XP “ipconfig” utility since this is available in the majority of Windows Operating Systems.

First
go to “start” -> “run” and type “cmd” without the quotes. Then hit the enter key. At the command line type “ipconfig /all”
again without the quotes. Actually
just typing ipconfig without the /all will work but will only provide you with abbreviated information regarding your network cards. An example of what you might see by typing the “ipconfig /all” command is below:

OutPut Of The “Ipconfig /All” Command
----------------------------------------------------------
Windows IP Configuration


Host Name . . . . . . . . . . . . : Home Computer
This is the name of your computer
typically defined during the windows installation. However
it can be changed after installation.

Primary Dns Suffix . . . . . . . : domain.com
If your computer participates in a network such as a Microsoft Windows domain this item may contain the name of the domain.

Node Type . . . . . . . . . . . . : Unknown
The Node Type may say Unknown
or peer-to-peer
or in some cases “hybrid”. It is a setting that has to do with the Windows Internet Naming Services used in certain types of Windows domain networks.

IP Routing Enabled. . . . . . . . : No
This setting determines if Windows XP or 2000 will function as an IP router. If you have two or more network cards you can setup your system to act as a router
forwarding communications requests from one network to another. Windows 2000 can be configured to do this in a pretty straight forward fashion; Windows XP will need a registry modification.

WINS Proxy Enabled. . . . . . . . : No
WINS Proxy is another setting that is related to the “Node Type” we discussed earlier. It is normally not a required setting in a home or small office network
or newer types of Microsoft Windows domains.

Ethernet adapter Wireless Network Connection 2:
If you have multiple Ethernet (network) cards in your systems
as I do in this laptop
you will have multiple listings. This one happens to be the second Ethernet card
an internal wireless Ethernet card.

Description . . . . . . . . . . . : Broadcom 802.11b/g WLAN
This is the description of the Ethernet card
usually the Name / Manufacturer and type of Ethernet card. In this case
it is a Broadcom wireless Ethernet card built into my laptop.

Physical Address. . . . . . . . . : 00-90-4B-F1-6E-4A
And here we have the MAC address. The MAC address is a 48 bit hexadecimal code and is suppose to be a totally unique address. It is 48 bits because each number or letter in hexadecimal represents 8 bits. Hexadecimal numbers range from 0
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F. There are 6 alpha-numeric codes hence 6*8=48(bits). The first 3 codes identify the manufacturer of the card and the remaining codes are used to create a unique number. Theoretically there should never be a card with same MAC address on a local network. However
there are a few exceptions. There are software tools that allow you to change this code. In fact
this is a step some hackers take to attack other systems on a local network. I say local network because MAC addresses are not routable between network segments. By spoofing this address
you can impersonate another machine on the local network. Traffic that was bound for the intended target can be redirected to the hacker’s machine. This is the address you would also use to populate a MAC address
or physical address table when setting up your wireless access point to support MAC address filtering.

DHCP Enabled. . . . . . . . . . . : Yes
DHCP
or the Dynamic Host Control Protocol
if enabled means your computers IP address is being provided by a DHCP server on you network. The DHCP server could be your wireless access point
cable/dsl router
cable modem
or a server on your network. Also
if a DHCP server is not enabled on your network
your computers Operating System will auto generate a random IP address within a certain predefined range. This means you could network a group of systems together without having to manually assign the IP settings.

IP Address. . . . . . . . . . . . : 192.168.0.117
This parameter provides you with your current IP address. The address listed above is what is called a "private" address. There are certain classes of IP addresses that have been set aside for private use. This means for your internal
local
or private network at home or office. These addresses are not
or should not
be routable on the Internet. The Internet routes what are called “valid” IP addresses. Your cable/dsl router or cable modem has a valid IP address assigned to its “external” network interface. The external interface may be your phone line or cable TV cable.

Subnet Mask . . . . . . . . . . . : 255.255.255.0
The Subnet Mask is a special number
or in some sense
filter
that breaks down your IP address
in this case private IP address
into certain groups. IP addresses and Subnet Masks can be a complicated matter and would take an entire article to go over.

Default Gateway . . . . . . . . . : 192.168.0.254
The default gateway
the IP addresses listed above
is the IP address of the device that will route your request
such as when you try to browse a website
to the Internet. It is a bit more complicated than that though as gateways or routers can route traffic to various different networks
even other private networks. At your home or small office
this gateway most likely is your cable/dsl modem or router.

DHCP Server . . . . . . . . . . . : 192.168.0.49
The DHCP server
remember we talked a little about this above
is the device that assigns your computer an IP address and other information. DHCP servers can assign all kinds of information such as; Default Gateway
Domain Name Servers (DNS)
IP address
Subnet Mask
Time Server
and much more.

DNS Servers . . . . . . . . . . . : 192.168.0.49
64.105
Servers are internal or external servers that resolve Fully Qualified Domain Names (FQDN)
such as www.defendingthenet.com
to IP addresses. This is done because computers don’t actually transmit your requests using the domain name
they use the IP address assigned to the FQDN. For most home or small office users
the primary DNS server is the IP address of your cable/dsl router. Your cable/dsl router than queries an external DNS server on the Internet to perform the actual resolution of the FQDN to IP address. The address 192.168.0.49 is an internal private device on my network whereas the 64.105.197.58 is an external public Internet DNS server and is present just in case my router has trouble performing the DNS resolution tasks.

Lease Obtained. . . . . . . . . . : Sunday
March 19
2006
6
PM
This information tells you when your computer received its IP address and other information from a DHCP server. You will notice it says “Lease Obtained”
that is because most DHCP servers only lease the IP address to you from a pool of available address. For instance
your pool may be 192.168.1.1 through 192.168.1.50. So your DHCP server has 50 IP addresses to choose from when assigning your computer its IP address.

Lease Expires . . . . . . . . . . : Wednesday
March 29
2006
9
PM
When the IP address
assigned by the DHCP server
lease expires it will attempt to lease you the same or another IP address. This function can typically be changed on the DHCP server. For instance
on some fully functional DHCP servers
you can configure the Lease to never expire
or to expire within 1 day and so on.

Why Are MAC Addresses So Important And How Do They Work
------------------------------------------------------
To jump back to MAC address for just a bit. You may think that IP addresses are the most important thing when it comes to network communication. The reality is
MAC addresses are very important because without them computers would not be able to communicate over Ethernet networks. When a computer wants to speak with another computer on a local network
it will make a broadcast request
or ask a question
of who owns a particular IP address. For instance
your computer may say “Who is 192.168.0.254”. Using the information above
my default gateway is 192.168.0.254 and will answer “I am “00-90-4B-F1-6E-4A” 192.168.0.254”. It sends back its MAC address. That MAC address then goes into what is called a Address Resolution Protocol (ARP) table on your computer. You can see this information by going to the command prompt like you did above and typing "arp –a". You will get information like the following:


Internet Address Physical Address Type
192.168.0.49 00-12-17-5c-a2-27 dynamic
192.168.0.109 00-12-17-5c-a2-27 dynamic
192.168.0.112 00-0c-76-93-94-b2 dynamic
192.168.0.254 00-0e-2e-2e-15-61 dynamic

How A Hacker Can Use MAC Addresses In An Attack
----------------------------------------------
You will notice the IP addresses and to the right of them the MAC addresses. Without this information
without the MAC address
you would not be reading this article right now. MAC addresses are not routable like IP addresses. They work on your local or private network. However
devices on the Internet perform the same tasks. Routers and switches maintain a list of their peer devices MAC address just like your computers and devices on your home or office network. I mentioned above that MAC addresses can be changed in order to redirect requests. For instance
if I were on your office network and you had an internal web server that took personal information as input
I could tell your computer to go to my laptop for the web site by broadcasting my MAC address tied to the real web servers IP address. I would do this when you computer asked “Who is the “Real Web Server””. I could setup a fake web server that looks just like the real thing
and start collecting information the real web server would normally collect. You can see how dangerous this can be.

Conclusion
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There are several other easy ways you can find your MAC address but they can be a little confusing if you have more than one internal network card. Most external USB
or PCMCIA wired and wireless Ethernet cards have their MAC address printed on them. In cases where the wired or wireless network card are inside your computer
such as in laptops
the MAC address is sometimes printed on the bottom of the laptop. Even Desktop systems cards that are inserted in PCI slots have the MAC address printed on the Ethernet card.

You may reprint or publish this article free of charge as long as the bylines are included.

Original URL (The Web version of the article)
------------
http://www.defendingthenet.com/NewsLetters/FindingYourMACAddressOnWiredAndWirelessNetworkCards.htm

About The Author
----------------
Darren Miller is an Information Security Consultant with over seventeen years experience. He has written many technology & security articles
some of which have been published in nationally circulated magazines & periodicals. If you would like to contact Darren you can e-mail him at Darren.Miller@defendingthenet.com. If you would like to know more about computer security please visit us at http://www.defendingthenet.com.