Multicast IP addresses
- x
- convention：
- Emotion(0)
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In the first octet, bit 0 has been reserved for broadcast or multicast traffic. When we have unicast traffic this bit will be set to 0. For broadcast or multicast traffic this bit will be set to 1. On layer 3 IANA has reserved the class D range (224.0.0.0 – 239.255.255.255) for multicast IP addresses.
For layer 2 we also have a reserved prefix to use for multicast traffic. The 24-bit MAC address prefix 01-00-5E is reserved for layer 2 multicast. Unfortunately only half of the MAC addresses in this 24-bit prefix can be used for multicast, this means we only have 23 bits of MAC address space to use for multicast. Here’s an illustration:
This means that we have to map multiple Multicast IP addresses to the same Multicast MAC address. We don’t have enough MAC addresses to give each multicast IP address its own MAC address.
We miss 5 bits of mapping information: 25 = 32. This means we will map 32 multicast IP addresses to 1 multicast MAC address. Here’s an example:
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In the first octet, bit 0 has been reserved for broadcast or multicast traffic. When we have unicast traffic this bit will be set to 0. For broadcast or multicast traffic this bit will be set to 1. On layer 3 IANA has reserved the class D range (224.0.0.0 – 239.255.255.255) for multicast IP addresses.
For layer 2 we also have a reserved prefix to use for multicast traffic. The 24-bit MAC address prefix 01-00-5E is reserved for layer 2 multicast. Unfortunately only half of the MAC addresses in this 24-bit prefix can be used for multicast, this means we only have 23 bits of MAC address space to use for multicast. Here’s an illustration:
This means that we have to map multiple Multicast IP addresses to the same Multicast MAC address. We don’t have enough MAC addresses to give each multicast IP address its own MAC address.
We miss 5 bits of mapping information: 25 = 32. This means we will map 32 multicast IP addresses to 1 multicast MAC address. Here’s an example:
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