GATE IT 2004 | Question: 27

Question

A host is connected to a Department network which is part of a University network. The University network, in turn, is part of the Internet. The largest network in which the Ethernet address of the host is unique is

• A. the subnet to which the host belongs
• B. the Department network
• C. the University network
• D. the Internet

Comments

In two different network two different devices can have same MAC right ? reffer : comment  

So answer should be Department Network

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Ethernet Address == MAC Address

MAC Address is Unique in entire World. It is encoded inside NIC(network interface card).

ans is (D) The internet

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resource 1
resource 2
resource 3 (very important)
This question is from 2004 and, answer was indeed D but..
See IEEE resource 3, in modern IOT days, we can have local mac addresses, which are unique locally only. 
As much as i could infer, if same question comes again, answer A should me marked.
 

Answer 1

Answer is D, Ethernet address is nothing but MAC Address which is present on NIC and it is unique for every network device (a single system might have multiple network cards and each can have its own MAC address).

PS: We can never say Ethernet address is unique only in a network -- because it is independent of the network a device is connected to. That is, if we move a device from one network to another, MAC address remains same. Of course we can do spoofing, but this is not relevant to the asked question.

Comments

@Sun Wukong You are wrong. There are devices that do not come with default MACs and assign one to themselves randomly when they are powered on. 

The point is that the standard requires for a MAC to be unique at the LAN level. While it has provisions for unique address spaces for organizations, it doens't require implementers to use these provisions, hence the devices that assign themselves random MAC addresses at boot.

The expectation that a NIC you buy from some organization should come with a unique MAC address is a organizational matter not a standard compliance matter. The uniqueness of the MAC addresses(or lack there of) is the responsibility of the organization who holds the block of addresses from which they assign your NIC one.

I implore you to read a little bit about this.

802.3-2001 (read section 9, this explains the usage of universal addresses)

802.3-2012 (read section 3)

How is uniqueness of MAC addresses enforced? the accepted answer to this is also a good read.

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What would be the case if we are talking about IP address?

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@Ozo  In that case more information is needed to answer. Most probablistic answer is (A) the subnet to which the host belongs But Host is connected to department network , So department network also be correct answer . In case of IP address the IP address is unique in the subnet in which it beongs to , which subnet is unique in the university , May be university allocated class be unique ( or classless adressing ) So , we can't conclude on IP addresses it may be ambiguous . 

Answer 2

The answer should be (D) since a specific LAN technology - Ethernet is mentioned here, so MAC addresses will be specifically taken as physical address which is unique in the entire world.!

If in general LAN Address was mentioned, then we would have had to take Physical Address concept, which only needs to be unique within the network, so answer would have been A in that case.

Comments

This answer is wrong. 802.3 networks(which includes ethernet and wifi) only require addresses to be unique amongst a network segment(LAN). They have explicit support for setting arbritary MAC addresses via the Universal/Local bit.

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Yes we can change MAC but it will be stored in RAM for temporary time , we can't change the actual value in the ROM Of NIC  assigned by manufacturer of our NIC , SO MAC is unique. 

unistall your window Or driver that is managing your changed MAC , devices will return to their default MAC. 

Answer 3

Answer is D.

Because the Ethernet address will be unique for every host.

Answer 4

Answer should be D.

As a media access control address (MAC address) of a device is a unique identifier assigned to a network interface controller (NIC) for communications at the data link layer of a network segment.

Source: https://en.m.wikipedia.org/wiki/MAC_address

Comments

It is required to be unique in a network segment. The smallest network segment is a LAN/subnet.

 

Answer 5

Answer should be A.

Ethernet is a LAN protocol. Which means that no two devices in a LAN can have the same MAC address. You can have two ethernet LANs connected via some network layer protocol each having a device with the same MAC address and everything would work.

Generally NICs have a on-rom MAC address but a lot of them have the ability to change it via the driver(in memory). All androids phones support this under MAC randomization.

802.3 standards have express support for this via universal/local bit. Which allows network administrators to set MAC addresses. See https://en.wikipedia.org/wiki/MAC_address#Universal_vs._local_(U/L_bit)

The largest network Ethernet address is required to be unique is a subnet/LAN. So the answer should be A.

Comments

option A is true when the largest network is not asked .

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No. Option A is the answer regardless. It has nothing to do with network size. MAC is only required to be unique for a router/lan. The only MAC addresses a router needs to know are the ones it is directly connected to(inculding hubs and switches that extend those connections). In internet there are multiple devices with the same MAC address, I know this because I have done this(i.e. setup two LANS connected to the internet and each have a device share MACs, you can also connect these together, which also works). It works flawlessly.

Answer 6

Ethernet standard is IEEE 802.3. 

It uses CSMA/CD (carrier sense multiple access with collision detection) access method with speed up to 100 Gbps

An Ethernet address, also known as a MAC (Media Access Control) address, is a 48-bit physical address that is hardcoded into a device's Network Interface Card (NIC) during manufacturing. According to the IEEE standards, these addresses are designed to be globally unique.

 the Ethernet address remains unique not just within its local subnet or university network, but across the entire Internet. 

Why the other options are incorrect:

• ❌ the subnet to which the host belongs: While the address is unique here, a subnet is the smallest network segment mentioned. The question asks for the largest network So false
• ❌ the Department network: This is a local network, typically composed of multiple subnets. Although the address is unique within it, the department is still a subset of the larger university and global networks.So false
• ❌ the University network: Like the department network, this is a private infrastructure. While the MAC address is unique within the university, the scope of its inherent uniqueness extends further to the global scale of the Internet.