IPv4 network addressing

IPv4 address 1. IPv4 packet header IP source IP address destination IP address TTL-the 8-bit TTL field describes the maximum number of hops that can pass before a packet is discarded or can be transmitted. After data packets are processed, each vro drops the TTL value by 1 and the data packet whose TTL value is 0 is discarded. Service type (ToS)-8-bit field, which describes the priority level used by the router to process data packets. Protocol-8-bit field, indicating the upper-layer protocol-such as TCP, UDP, or ICMP-unencapsulate and receive data packets and hand them over to the transport layer. Flag and shard offset-when a router transfers data packets from one media set to another MTU (the smallest transmission unit, which is an attribute of the media set), data packets may be sliced. After a shard occurs, the packet uses the segment offset and flag in the IP header to recreate the packet. The part offset field specifies the order in which the data packet shards are rebuilt. Version -- indicates that the IP version number is 4 or 6. header length (IHL) -- because it contains the option field, the length is not the same. Packet Length-the entire packet length, including the header. The minimum length of a data packet is 20 bytes (no data is included in the header) and the maximum length is 65535 bytes. ID-all parts are re-created by the source. Header checksum-used to specify the header length. Each vro in the path will check the header length. Run an algorithm. If the checksum is invalid, the packet is discarded if it is damaged. Because each vro needs to modify the TTL value, the header checksum and re-calculation are performed on each hop. Option-some fields have been prepared separately for other services in the IPv4 header, but these fields are rarely used. Fill-when the header data is not an integer multiple of 32 bits, it needs to be filled. 2. Communication compilation type: unicast, broadcast, and multicasting are in the network. The host can communicate in one of the following three ways: 1. unicast-one-to-one transmission packet address 0.0.0.0 to 223.255.255.255 is used for the host address. 2. Broadcast: send data packet broadcast and Multicast from a host to all hosts in the network. This address is called a broadcast address. Broadcast is divided into two types with different Compiling Methods: Targeted broadcast-targeted broadcast refers to sending data packets to all hosts in a specific network (non-local network. Targeted broadcast uses the largest IPv4 address in the network. For example, when an external host needs to communicate with a host in the network 172.16.4.0/24, the destination address of the packet should be 172.16.4.255. Limited broadcast-limited broadcast is limited to sending packets to hosts on the local network. The destination address is 255.255.255.255. 3. multicasting-data packet multicast transmission from a host to a selected group of hosts is designed to save network bandwidth. If unicast communication is used to communicate with multiple target hosts, the source host needs to send data packets to each host one by one. However, if multicast transmission is used, the source host only needs to send one packet to communicate with all target hosts. The IPv4 address from 224.0.0.0 to 239.255.255.255 is reserved for multicast communication. Multicast addresses are classified into different types. 224.0.0.0 to 224.0.0.255 is the local address of the reserved link. These addresses are mainly used for Multicast Groups in the network. The TTL of the destination data packet from this address is generally 1. 224.0.1.0 to 238.255.255.255, which is a global address used to transmit multicast data over the Internet. For example, 224.0.1.1 is reserved for the network clock protocol, which is used to synchronize the clock of the network device. 3. reserved and special IPv4 address multicast address: the front of the IP address is "1110", so the IPv4 multicast address range is 224.0.0.0 to 239.255.255.255. Private IP Address: 10.0.0.0/8 to 10.20.255.255/8; 172.16.0.0/12 to 172.31.255.255/12; 192.168.0.0/16 to 192.168.255.255/16. Private addresses are used in private networks. Data packets using these addresses as the source and destination should not appear on the public network. Whether a data packet exists on the Internet is the main difference between a private address and a shared address. Vrouters or firewall devices on these private network boundaries must block or convert (to a public address) such addresses. Default route: the default route is to forward data packets to the default route when no available path is found in the route table. The default IPv4 route address is 0.0.0.0. This address retains all the addresses from 0.0.0.0 to 0.20.255.255. Loop (send) Address: the address that the host uses to communicate with itself is the loop address. The IPv4 loopback address is 127.0.0.1. (The addresses 127.0.0.0 to 127.255.255.255 are retained for loop test .) Local link address: when no IP address is available, the address assigned to the host by the operating system is the local link address. It can be used for point-to-point networks or when the host cannot automatically obtain the address from the Dynamic Host Configuration Protocol (DHCP) server. The local link address blocks are 169.254.0.0 to 169.254.255.255. Test-Network Address: address blocks from 192.0.2.0 to 192.0.2.255 are used for teaching purposes. These addresses can be used in documents and network examples. 4. ping and tracert tool ping is a program used to test IP connectivity between hosts. Command example: pingwww.163.com where command ping 127.1 (127.0.0.1) is used to test the host loopback and check whether the TCP/IP on the host is correctly installed. The tracert tool is used to observe the path between two hosts. For example, test the path from my host to www.csdn.net. 222.185.127.81 is the first vro for data packets. So even if the gateway address of the network where the host is located.