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Mac Address 基础知识

2010年5月11日 / 32次阅读  InfoTech

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Mac Address有48bit。(IPv4 address is 32bits. IPv6 address is 128bits.)

单播Mac Address:

mac address结构

mac address结构

Broadcast Mac: FF-FF-FF-FF-FF-FF.

Multicast Mac: The high order 25 bits (01-00-5e-) of the 48-bit MAC address are fixed and the low order 23 bits are variable.

mac address 组播地址映射

mac address 组播地址映射

To map an IP multicast address to a MAC-layer multicast address, the low order 23 bits of the IP multicast address are mapped directly to the low order 23 bits in the MAC-layer multicast address. Because the first 4 bits of an IP multicast address are fixed according to the class D convention, there are 5 bits in the IP multicast address that do not map to the MAC-layer multicast address. Therefore, it is possible for a host to receive MAC-layer multicast packets for groups to which it does not belong. However, these packets are dropped by IP once the destination IP address is determined.

【交换机如果不运行IGMP Snooping,对于组播MAC报文来说,就是广播】

这样的映射有何作用?

2层交换机通过此地址格式来判断接收到的frame类型,单播frame直接查找mac转发表,组播报文查找通过IGMP snooping建立起来的组播转发表,这样组播就是组播,不用广播,优化性能。技术上可以解决组播IP地址后23bit映射带来的mac组播地址重复,这就要2层交换机snooppacket层面,建立的组播转发表也是基于组播IP地址的,而不是组播Mac地址,但是带来的就是转发性能的降低,不过减少了无效转发。可以考虑一个折中的办法:只对会造成MAC组播地址重复的组播IP地址进行特别处理。】

Mac address is also referred to as hardware address, which is burned into the network interface card.

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