A number to identify devices on the network. In IPv4, it is represented by 32 bits, and it is common to divide it into 8 bits and represent it in 4 fields for human readability. It is divided into a network part (which network it belongs to) and a host part (which computer terminal), and by looking at the first pattern, you can see how far the network part is.
|Class A||0.0.0.0〜127.255.255.255||The network part has only the first 8 bits. The host can be represented using the last 24 bits.|
|Class B||220.127.116.11.～18.104.22.168||The network part is the first 16 bits.|
|Class C||192.0.0.0.～22.214.171.124||The network part is the first 24 bits.|
|Class D||126.96.36.199〜188.8.131.52||The network part is the first 24 bits.|
With conventional classful addresses, it was necessary to provide some bits of the IP address (24 bits for class C) as the network part. As it became clear that IP addresses were exhausted as the network deepened, a method using numbers called subnet masks was adopted. The subnet mask is a numerical value that distinguishes between the network part and the host part.
--One part of the subnet mask --Network part of IP address --The 0 part of the subnet mask --IP address host part
There are two types of IP addresses, global and local. Devices connected to www (eg routers) do not know where in the world they send or receive data, so they need to have the only IP address in the world. By doing this, you can communicate in a fixed manner from within www. However, a PC hanging below a router only needs to have a number that can be identified within the local network, which is a private address.
It can be further divided into two types of IP addresses
--Dynamic IP address --The ISP automatically assigns a global address. It changes to a different IP address for each connection and each time limit. --Static IP address --A fixed IP address. Used for homepage public servers, mail servers, etc.
Private addresses are automatically assigned by the DHCP server in the home. Since it is built into a broadband router, it does not need to be prepared by the user.
The receiving side opens the port and waits. By doing this, when the destination delivers the packet to the server (or personal computer) by relying on the IP address, it is clearly indicated to which application it should be passed. Port numbers are roughly divided into three types: well-known ports, registered ports, and dynamic ports. Even during socket communication, it is necessary to clearly indicate what port number is open and waiting.
Communication that sends the same data from one source to multiple destinations. A class D destination IP address is used. It is used for real-time video distribution and OS image distribution to PCs. Instead of sending 100 packets to 100 destinations, one multicast packet is sent to improve efficiency.
|type||Formal name||Japan name||Description|
|DNS||Domain Name System||Domain name system||Convert website names to IP addresses and vice versa|
|DHCP||Dynamic Host Configuration Protocol||Dynamic host configuration protocol||A protocol that helps you assign IP addresses and associated IP information to computers in your network|
|Proxy server||Relay server|
A server that operates on the border between the internal network and the Internet and assists in access. By connecting the personal computer to the network via a proxy server instead of directly connecting to the Internet, it is possible to prevent unauthorized access and intrusion and improve security. In addition, since the proxy server has a cache function, it is possible to temporarily save the browsing history of the Web.
A device for evenly distributing the load on the server. A device that is also necessary to distribute the load for the operation of websites that are frequently accessed.
A method that does not use a load balancer, which is one of load balancing. It can be configured by writing the setting information in advance to the setting file of the DNS server. The DSN has a function to associate the domain name (name of the Web page) with the IP address assigned to the server, and it is possible to record which server the user is using. We will use this to allocate users. However, there are some disadvantages and the flexibility is low.
|Horizontal function distribution||All PCs on the network are connected equally|
|Vertical function distribution||There is a master-slave relationship between PCs on the network|
When Server A and Server B are connected in series. In the first place, all two PCs need to be running,
\epsilon_A \times \epsilon_B
It is expressed in the form of a product.
When Server A and Server B are connected in parallel. The probability that server A will not run is $ 1- \ epsilon_A $, and the probability that server B will not run is $ 1- \ epsilon_B $. Therefore, subtracting the probability that the system will not function at all from the overall probability represents the system utilization rate, so in the case of parallel
1-(1-\epsilon_A) \times (1-\epsilon_B)
It is expressed in the form of.
|Fail soft||Continue operation by reducing the processing capacity of performance and functions|
|Fallback||Detach the location of the failure and reduce the capacity to drive|
|fail safe||Move safely in the event of a failure|
|Fault tolerant||Always keep the system running normally|
|Foolproof||Pre-designed measures against typos and erroneous operations|
RAID (Redundant Arrays of Inexpensive Disks) systems are commonly used. Technology for connecting multiple hard disks to provide redundancy and protect important data from physical failures.
|RAID0 (striping)||Data is distributed and written to multiple disks. Redundancy like RAID5 is not planned.|
|RAID1(Mirroring)||Two disks are connected to one disk controller, and the data written on one disk is automatically reflected on the other disk.|
|RAID2〜４||Since the RAID5 method is generally used, it is not used in a realistic system.|
|RAID5||Data and parity information are distributed and written to multiple disks. If one disk fails, it can be restored from the rest of the data and parity information.|
|RAID10||It is composed of RAID0 and RAID1. Further redundancy can be ensured, but the number of HDDs that must be used increases, which is costly.|
RAID0 Distribute one data (for example, half each) and write it to two (or multiple) HDDs. The RAID controller automatically divides the data and writes it to each drive. Therefore, it is possible to increase the write processing speed, but be aware that if a problem occurs with one HDD, all data cannot be read.
RAID1 Write exactly the same data to two (or multiple) HDDs. In the case of data deletion, it will be deleted from the two HDDs, so it is necessary to keep a backup separately. However, in normal operation, even if one of the HDDs fails, it can be read from the other, so backup is taken in that sense.
RAID5 While dividing the data like RAID0, writing is performed while generating a code (parity) that corrects an error for data recovery. Therefore, data can be read even if one HDD fails, but if two or more HDDs are damaged, the data will be lost. Therefore, it is necessary to promptly repair one HDD when a failure occurs. RAID6 can withstand problems with two HDDs, but the usable disk space is reduced accordingly.
A system that has a function to keep a journal (update history) and prevent damage to the management area when changing the management area.
NAS Storage that can be connected to a network (Network attached strage). It's like a cloud server, so you can easily share files and browse files when you're at home.
When the memory usage exceeds the actual amount of physical memory, it is temporarily written to the unused HDD area. This is called a swap, and that area is called a swap area. When swap occurs, it leads to an increase in writing to the HDD, which causes a decrease in operating speed. It can be displayed with the
$ free total used free shared buff/cache available Mem: 29987936 7728424 8926592 1425036 13332920 20442572 Swap: 10485756 7895908 2589848
Swap areas can be created and activated with the
Firewall A system that detects unauthorized access and notifies the administrator.
--Packet filtering --Analyze the header to determine whether to pass (communicate) --Powerful and flexible, but difficult to set up and likely to create security holes --Application level gateway --Connect via proxy server and authenticate for each service --It is relatively easy to set up, but you cannot control it in detail.
WAF（Web Application Firewall）