A host subscribing to the multicast stream of 220.127.116.11 would map to MAC address of 0x0100.5E40.0707, and so would 18.104.22.168 and 22.214.171.124. It all boils down to 1s and 0s. Figure 2-5 shows the ambiguity. The “X” in the binary row represents the bits that are overwritten and shows how 32 multicast IP addresses map to a single multicast.
This tutorial explains the MAC (Media Access Control) address in detail. Learn what the MAC address is, how it is formed, and the types of MAC addresses (unicast, multicast, and broadcast).
In network, an address provides a unique identity to an end device. Unless an end device has a unique address, it can’t communicate with other devices in the network. A unique address enables an end device to send and receive data in the network.
In the LAN network, a unique address is the combination of two addresses; software address and hardware address.
Addressing in Networking Reference models
A networking reference model defines the standards, characteristics, definitions, and functionalities of the network. There are two popular networking models; the OSI Seven Layers model and the TCP/IP model.
In both models, the software address and hardware address are defined in the network layer and data link layer, respectively. In both models, the network layer and data link layer stand on the third and second positions, respectively. Because of this, both layers are also known as layer 3 and layer 2, respectively.
This MAC address is congruent with both these multicast IP addresses, so if 2960 and 3560 based their forwarding on MAC addresses, the host subscribed for 126.96.36.199 would be receiving even the stream to 188.8.131.52. However, the receiver did not receive any multicast packets of this stream. Jan 19, 2019 When a client on the local segment needs to send a multicast message, it will use a Frame with a special MAC address. Multicast addresses within a MAC address always begin with 01-00-5E. The rest of the MAC address is a changed format, derived from the multicast IP address. Figure 51: Mapping of Multicast IP Addresses to IEEE 802 Multicast MAC Addresses. IP multicast addresses consist of the bit string “1110” followed by a 28-bit multicast group address. To create a 48-bit multicast IEEE 802 (Ethernet) address, the top 24 bits are filled in with the IANA’s multicast OUI, 01-00-5E, the 25th bit is zero,. Oct 29, 2010 By looking on the form of the MAC address, it seems that the MAC address corresponds to one of the following 32 possible IP addresses: 184.108.40.206 220.127.116.11.
The software address is also known as the network layer address or layer 3 address. This address is manageable and configurable. Based on network requirements and layout, this address can be configured and assigned to an end device. Almost all modern LAN implementations use the IP protocol in the network layer. The IP protocol uses the term IP address to define the software address.
I have already explained IP addresses in the following tutorial.
In this tutorial, I will explain the hardware addresses in detail.
The hardware address is also known as the data link layer address or layer 2 address or MAC (Media Access Control) address. From these terms, the term MAC address is commonly used to refer to the hardware address. Unlike the IP address or software address, this address can’t be configured or managed. When you purchase a new NIC (Network Interface Card), or any device which has onboard NICs, it comes with a pre-configured MAC address.
A MAC address is 6 bytes (48 bits) long address in the binary numbers. MAC addresses are written in the hexadecimal format. The hexadecimal format uses the base-16 to refer to numbers. If we divide the total available length (48 bits) in binary numbers by the base (base-16) that is used to write a number in hexadecimal format, we get the total digits (12 = 48 ÷ 16) of that number in the hexadecimal format. Thus, if we write a 6 bytes (48bits) long binary MAC address in hexadecimal format, we get a 12 digits long hexadecimal number.
For convenience and easier readability, when writing a MAC address in hexadecimal format, extra space or periods or colons are added after every two or four digits. For example, you can write a MAC address in the following ways.
- Without any separator: - 00000ABB28FC
- Extra space after every two digits: - 00 00 0A BB 28 FC
- Extra space after every four digits: - 0000 0ABB 28FC
- Colon after every two digits: - 00:00:0A:BB:28:FC
- Colon after every four digits: - 0000:0ABB:28FC
- Period after every two digits: - 00.00.0A.BB.28.FC
- Period after every four digits: - 0000.0ABB.28FC
No matter which style you use to write the MAC address, or an application or networking software uses to display the MAC address, a MAC address is always processed in binary numbers only. NIC converts hexadecimal numbers of the MAC address in binary numbers before processing and using it.
Structure or format of the MAC address
As mentioned above, you can’t assign MAC address to a NIC or onboard NICs. When you purchase a new NIC or a device with onboard NICs, it arrives with a pre-configured MAC address or MAC addresses, respectively. Before we understand how manufacturers select MAC addresses for NICs, let’s briefly understand why a MAC should be unique in the LAN network.
If a LAN network has two or more NICs configured with the same MAC address then that network will not work. Let’s understand this with an example.
Suppose in a network three PCs; PC-A (11000ABB28FC), PC-B (00000ABB28FC) and PC-C (00000ABB28FC) are connected through a switch. NICs of PC-B and PC-C have the same MAC address 00000ABB28FC.
If PC-A sends a frame to the destination MAC address 00000ABB28FC, the switch fails to deliver this frame as it has two recipients of this frame.
Mac Address For Multicast Ip Address Settings
The following image shows this example.
A LAN network does not work unless each device in the LAN network has a unique MAC address.
Now let's be back to our main question. How do manufacturers assign a unique MAC address to each NIC?
Before manufacturing NICs, every manufacturer obtains a universally unique 3-byte code, known as the organizationally unique identifier (OUI), from the IEEE. The IEEE is an international organization that regulates and maintains the namespace of MAC addresses.
After obtaining the OUI bytes, the manufacturer uses these OUI bytes at the beginning of the MAC address of all its NICs or on-board NIC devices. The manufacturer also assigns a unique hexadecimal value in the remaining bytes.
6 bytes MAC address = 3 bytes OUI number obtained from the IEEE + 3 bytes unique number assigned by the manufacturer
MAC addresses of all NICs or onboard NIC devices manufactured by the same manufacturer always start with the same 3-bytes OUI numbers. For example, suppose the IEEE assigns an OUI “0000AA” to the xyz company. Now the xyz company will use the OUI number 0000AA as the first 24 bits to build MAC addresses for its NICs or onboard NICs devices.
To keep each product separately from others, the manufacturer uses the remaining 3-bytes. Manufacturers are free to use any sequence or method on the remaining three bytes. For example, the xyz company can assign the MAC addresses to its NICs in the incremental order.
The following table extends this example and adds two more demo companies (ABC and JKL) in the example. It also shows MAC addresses of 5 NICs from each company.
Thus, this procedure ensures that no two NICs use the same MAC address in the universe.
Types of MAC address
There are three types of MAC address; unicast, multicast, and broadcast.
Unicast MAC address
Unicast MAC address represents a specific NIC or onboard NIC ports in the network. The inbuilt MAC address of a NIC is the unicast MAC address of that NIC.
Multicast MAC address
Multicast MAC address represents a group of devices (or NICs in Layer 2). The IEEE has reserved the OUI 01-00-5E (first 3-bytes or 24 bits) for the multicast MAC addresses. The remaining 24 bits are set by the network application or device that wants to send data in the group. A multicast MAC address always starts with the prefix 01-00-5E.
Broadcast MAC address
Broadcast MAC address represents all devices in the network. The IEEE has reserved the address FFFF.FFFF.FFFF as the broadcast MAC address. Any device that wants to send the data to all devices of the network, can use this address as the destination MAC address.
Mac Address For Multicast Ip Address Format
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