Explanation of TCPIP (11) Broadcast and Multicast

 

1. Introduction

Unicast transfers data to a specific host. If an IP packet is sent to a host, the link layer header is a very specific destination address. For Ethernet, It is the MAC address of the NIC.

Broadcast and multicast are only used in UDP. They are very important for applications that need to transmit packets to multiple receivers at the same time.

Broadcast is to send packets to all hosts on a network. The network may be a network, a subnet, or all subnets. If it is a network, for example, the broadcast of a type of web site is netid.255.255.255. If it is a subnet, It is netid. netid. subnetid.255; for all subnets (Class B IP addresses), it is netid. netid.255.255. The MAC address FF-FF used by the broadcast. All hosts in the network will receive the broadcast data, the network card as long as the MAC address for the FF-FF-FF-FF-FF-FF of the data to the kernel can be. Generally, ARP or the route protocol rip should be broadcast.

Multicast sends data to a specific host (Multicast Group). In this way, the data broadcast range is smaller.

2. Broadcast

(1) Restricted broadcast: The restricted broadcast address is 255.255.255.255.255. This address is used for the destination address of the I p datagram during host configuration. At this time, the host may not know the network mask of its network, or even its I P address. In any case, the vro does not forward a datagram whose destination address is a restricted broadcast address. Such a datagram only appears in the local network.

(2) Class A website broadcast: netid.255.255.255

(3) subnet broadcast: the address that points to the subnet's broadcast address. The master machine number is full 1 and has a special subnet number.

(4) broadcast pointing to all subnets: The subnet number pointing to the broadcast addresses of all subnets and the host number being all 1

Broadcast is a function that should be used with caution. In many cases, I p multicast has proved to be a better solution.

3. Multicast

The MAC address of multicast is the lowest byte level, for example, 01-00-00-00-00-00-00. The multicast group address is a Class d ip address, which must be 224.0.0.0-239.255.255.255. The ethernet address corresponding to IP multicast ranges from 0 1: 0 0: 5 E: 0 0: 0 0: 0 0 to 01: 00: 5E: 7f: FF: ff.

By ing its low position of 23 bits to the corresponding ethernet address, you can convert multicast addresses to Ethernet addresses. Because address ing is not unique, other protocols are required to implement additional datagram filtering.

Several special multicast addresses:

1. 224.0.0.1 -- all system groups in the subnet.

2. 224.0.0.2 -- all routers in the subnet.

3. 224.0.1.1 -- the network implements the dedicated ntp ip.

4. 224.0.0.9 -- r00002 private IP Address