Computer network summary
1. P110 indicates the MAC frame format of the V2 format, where the data field (ie IP packet) length ranges in 46 bytes -1500 bytes, this range is defined for Ethernet, "1500 bytes" is also called Ethernet MTU (maximum transmission unit, the Ethernet defines the maximum length of the IP datagram to 1500 bytes). P185 indicates the format of the IP datagram, the "Total Length Field" is the sum of the header and the data part. The "total length field" is 16bit, so the maximum length of IP datagram can be 65535 bytes (216), but this range obviously exceeds 1500 bytes. So in the Ethernet, although the use of as long as possible, the transmission rate is increased, but due to the general application of the Ethernet, the length of the datagram cannot exceed 1500 bytes, and sometimes the length of IP datagram is still limited to 576 words. Section. When the length of the datagram exceeds the maximum transfer unit MTU (MTU is 1500 bytes in the Ethernet), it must be transmitted in the network after segmentation of the excessive data. At this time, the "Total Length Field" in the header of the slice data is the length of the IP datagram after fragmentation.
2. The "Type Field" in the MAC frame indicates which network protocol used to use the upper layer of the link layer. For example, when the value is 0x0800, it means that the upper layer uses the IP protocol, which is 0x1837, the upper layer uses the Novell IPX protocol.
3. Differences between IP Address and Hardware Address (Mac):
The physical address is the address used by the data link layer and the physical layer, and the IP address is the address used by the network layer and the above layer.
As can be seen from Fig. 6-11, the first part of the link layer below is the MAC address, while the IP layer and the above layer are IP address.
4. The actual network transmission is to be encapsulated into the MAC frame, and the first part of the MAC frame is the MAC address representation. The IP newspaper is an IP address representation, so the IP report is installed into a MAC frame. , You must refer to the ARP address mapping table, which is the source of ARP.
5. ARP is to solve the IP address of the "same LAN" host or router corresponding to the IP address and MAC address. "The same LAN" is the so-called "neighbor" in the Linux core. There is an IP and MAC address mapping of this local domain router in the ARP mapping table, with the aim of connecting to other nets from this LAN.
6. The process of sending and receiving is shown in P183, it is worth noting that [Receiver] will extract [sender's IP and MAC address] to [Mind Cache] to the [ARP Cache], so that the acceptor is sent next time When you are, you don't have to broadcast Mac.
