IPV4 addressing and subnetting of "computer network"

Before discussing IP addressing, we need to discuss how the host and router are connected to the network. A host usually has only one link linked to the network, and when the IP in the host wants to send a datagram, it is sent on the link, and the boundary between the host and the physical link is called the Interface (interface). Now consider a set of routers and their interfaces. Because the router's task is to receive datagrams from the link and forward those datagrams from some other link, the router must have two or more links linked to it. The boundary between a router and any of its links is also called an interface. Therefore, one router has multiple interfaces, and each router has a link. Because each host and router can send and receive IP datagrams, IP requires that each host and router interface have its own IP address. Therefore, an IP address is technically associated with an interface, not the host or router that includes the interface.

Each IP address has a length of 32 bits (equal to 4 bytes), so there is a total of approximately 40 (232) billion possible IP addresses. These addresses are generally written in the so-called dotted-decimal notation (dotted-decimal notation), where each byte in the address is written in decimal form, separated by a period (point) between the bytes. For example, the binary notation for address 193.32.216.9 is: 11000001 00100000 11011000 00001001. In the global Internet, each interface on each host and router must have a globally unique IP address, except after the NAT.

The traditional IP address is the address of the classification. Divided into a, B, C, D, e five categories. Regardless of the type of IP address, the network number and host number are composed of two parts. Where the network number represents the network to which the host is connected, and the host number represents the address in that host or router.

The IP addresses of the classifications are as follows:

Special IP Address:

There are some IP addresses that are used for special purposes, not as host IP addresses:

　　

　　Related to subnets:

　　The host number is 0 to indicate the network itself. For example, in a 192.168.0.0/24 subnet, 192.168.0.0 represents the subnet itself.

　　The host number is 1 to represent the broadcast address of this network. For example, in a 192.168.0.0/24 subnet, 192.168.0.255 represents the broadcast address of the subnet.

The 0.0.0.0 represents the entire TCP/IP network.

255.255.255.255 represents the broadcast address of the entire TCP/IP network. For this unit, this address refers to all hosts within the network segment (the same broadcast domain). This address cannot be forwarded by the router.

　　Reserved address:

Class A: 127.x.x.x (127.0.0.0~127.255.255.255) is reserved for the loopback interface. As a rule, most systems assign IP address 127.0.0.1 to this interface and name it localhost. An IP datagram passed to the loopback interface cannot be on any network.

Class B: 169.42.x.x if the host uses the Dynamic Host Provisioning Protocol (PROTOCOL, DHCP) function to obtain an IP address automatically, then when the Dynamic Host Setting protocol server (DHCP server) fails, Or the response time is too long to exceed a system-specified time, the system assigns such an address. If the host IP address is found to be one such address, the host's network is mostly not functioning properly.

　Private Address:

The private IP address value is used for the LAN, not for the WLAN connection. Therefore, private IP addresses cannot be used directly on the Internet, and the private IP address must be converted to a legitimate global IP address on the Internet through the gateway justification before it can be used for the Internet. The private IP address effectively solves the problem of insufficient IP address, the private IP address network segment is as follows:

Class A: 10.0.0.0 ~ 10.255.255.255

Class B: 172.16.0.0 ~ 172.31.255.255

Class C: 192.168.0.0 ~ 192.168.255.255

CIDR:

CIDR (Classless Inter-domain routing, Classless interdomain Routing) is a network partition method of IP address, which is based on the use of variable eldest son netmask (VLSM) to eliminate the division of a, B and C networks, and can realize the network structure with the support of software. This can greatly improve the utilization of IP address space, and reduce the size of the routing table, improve router routing and forwarding capabilities.

Note that routing protocols can be categorized into class-routing protocols and non-class routing protocols. The subnet mask is not included in packets that have class routing protocols. So only the non-class routing protocol (RIPV2, BGP,OSPF) can support CIDR.

The main features of CIDR are as follows:

1) eliminates the traditional concept of a, B, C-class address and subnet. This IP address is divided into two parts: network prefix, host number.

CIDR uses the slash notation, which is the number of bits that the IP address/network prefix occupies. Where the network prefix occupies the number of bits corresponding to the network number portion, equivalent to the subnet mask in 1 consecutive parts.

2) combine the successive IP addresses of the same network prefix into "CIDR address blocks". A CIDR address block can include multiple A, B, and C addresses, an aggregation of which is called a route aggregation, or a form of an over-grid. Route aggregation enables an item in the routing table to represent a number of routes from traditional classified addresses, which facilitates the exchange of routing information between routes, thereby improving network performance.

Resources:

1. "Computer network-top-down Approach" (fourth edition) 2.7/4.4

2. "Benevolent Programmer"

3. http://baike.baidu.com/view/39496.htm

4. http://baike.baidu.com/view/2558390.htm

IPV4 addressing and subnetting of "computer network"