If you will be using the more advanced features of our products, such as tunneling, you will need to understand the basics of how the Internet is structured. This guide is an attempt at relaying some of that understanding on part1.
- IP addresses
- DNS names
- Subnets
- Types of IP addresses and subnets
- TCP and UDP
- Direction of TCP connections
IP addresses
Every computer connected to the internet has an Internet Protocol or IP address which identifies the computer on the internet. In the currently most widely used version of the Internet Protocol – version 4 – IP addresses are 4 bytes long and are expressed in the form nn.nn.nn.nn. Each nn is a number between 0 and 255. When you connect to a web server to browse a web page, the DNS name of the web server, e.g. www.bitvise.com, is automatically translated by the software in your machine to an IP address in the nn.nn.nn.nn form. This address is then used to connect to the actual web server.
For example, the IP address of the server hosting www.globalssh.com at the time of this writing is 112.215.52.66. Our primary website, on the other hand, is hosted on several servers, and their IP addresses 107.154.85.120.
In a Windows Command Prompt session, you can discover the IP addresses associated with DNS names using the nslookup command: e.g. ‘nslookup www.globalssh.com’.
DNS names
IP addresses are difficult to remember, so the internet provides a translation service which translates memorable names into associated IP addresses. This facility is called the Domain Name System or DNS. You use DNS implicitly every time you type in an address such as ‘www.globalsshcom’ – your browser asks your operating system for translation into an IP address, and the operating system either returns a cached result, or inquires with a DNS server operated by your ISP. This server in turn either returns a cached result or inquires with another DNS server.
Subnets
No computer is directly connected to every other computer on the internet. Instead, each computer is a member of one or more subnets. Subnets, in turn, are connected to each other by machines called routers or gateways, which belong to multiple subnets, forwarding internet traffic from one subnet to the other and reverse. In order to successfully communicate with other computers throughout the internet, your computer must know what subnet it is part of, so that it knows what IP addresses are outside your local subnet and must be relayed through the gateway. In addition, your computer must of course also know the IP address of the gateway.
Typically, a subnet is a group of consecutive IP addresses, such as all IP addresses from 11.22.33.0 to 11.22.33.255. This is commonly expressed in either of two formats:
- The subnet mask format. Here, the subnet is expressed as 11.22.33.0 with subnet mask 255.255.255.0. The subnet mask indicates what bits of the subnet IP address indicate the actual subnet, and what bits are variable, indicating individual computers in the subnet. A byte consists of 8 bits, and 255 is 1111 1111 in binary. Therefore, 255.255.255.0 means that the first 3 bytes of the subnet IP address (11.22.33) indicate the actual subnet, and the last byte can be variable (and indicates computers in the subnet). If the subnet mask were 255.255.0.0, that would mean that the last two bytes are variable.
- The significant bits format. Here, the subnet is expressed as 11.22.33.0/24, which means subnet 11.22.33.0 with 24 significant bits. The 24 means that the first 24 bits of the subnet mask are 1, and all the following bits are 0. Thus, /24 is equivalent to a subnet mask of 255.255.255.0. /16 is equivalent to a subnet mask of 255.255.0.0. And because there are just 32 bits in an IP address, /32 indicates an IP address with no variable part: a fixed, constant IP address.
Types of IP addresses and subnets
There are three major types of IP addresses (or subnets) that you need to be aware of.
- Public IP addresses. Most IP addresses in the 32-bit address range have the purpose of uniquely identifying a computer on the internet. The IP address 112.215.52.66, for example, is a public IP address that uniquely identifies one of the servers hosting the www.globalssh.com website (and others). This is the type of IP address through which a server must be reachable in order to be accessible to computers throughout the internet.
- Private subnets. Special ranges of the 32-bit IP address range have been set aside for use in private networks, where the computers in such a network do not need to be directly accessible from the internet as servers (but may nevertheless access the internet through a gateway, as clients). These ranges include:
10.0.0.0/8 (addresses from 10.0.0.0 to 10.255.255.255)
172.16.0.0/12 (addresses from 172.16.0.0 to 172.31.255.255)
192.168.0.0/16 (addresses from 192.168.0.0 to 192.168.255.255) - Special IP ranges. There are several special purpose IP ranges, but the one you need to know about is 127.0.0.0/8 (addresses from 127.0.0.0 to 127.255.255.255). This is the local loopback range and is used to connect two programs running on the same machine. Any address in this range can be used for this kind of purpose, but the most commonly used are 127.0.0.1 and 127.0.0.2. The special DNS name ‘localhost’ translates to 127.0.0.1.
TCP and UDP
The Internet Protocol itself is a relatively rudimentary protocol which provides only the capability of delivering small chunks of data to other computers. The Internet Protocol does not provide reliability: chunks of data that are sent using the Internet Protocol may be lost. They also may arrive in an order different to the order in which the chunks were sent.
For some types of data transfer, the (un)reliability afforded by the Internet Protocol is fine. When streaming video, for example, it does not matter if chunks that make up intermediate frames of the video are lost. What matters is that most of the data arrives relatively quickly, allowing the video to be played with reasonable quality and on the fly. The User Datagram Protocol, or UDP, is a simple protocol layered on top of the Internet Protocol that provides this level of reliability. UDP is used for purposes such as relaying video and audio streams as well as for networked games; all environments where responsiveness and fast delivery are more important than perfect reliability.
For other types of data transfer, however, this level of reliability is not enough. When transferring a file, for example, you want to transfer all of its contents in perfect order and integrity; you don’t want any chunks of it to accidentally be lost. When accessing a web page, likewise, you want all the text to be transferred without error. Data transfers that require this higher level of reliability use the Transmission Control Protocol, or TCP. Like UDP, TCP is a protocol layered on top of the Internet Protocol, but it is more complex than UDP: it contains mechanisms to ensure that data is received in order and that, if any chunks are lost, they are resent. The reliability provided by TCP has costs in terms of responsiveness. Before any data can be sent using TCP, the two computers must engage in a short back-to-forth to establish a TCP connection. If any data are lost during transmission, delivery of subsequent data awaits until the data that were lost are retransmitted and delivered. When there is a high rate of data loss on a connection, this may cause transmission to be jerky.
The majority of widely known protocols used on the internet are layered on top of TCP. These include:
- the Simple Mail Transfer Protocol (SMTP), used for email delivery;
- the Post Office Protocol (POP) and IMAP, used for email retrieval;
- the Hypertext Transfer Protocol (HTTP), used for accessing websites;
- as well as, of course, the Secure Shell protocol (SSH).
Direction of TCP connections
TCP connections are like phone calls: they are always initiated by one party and accepted (or not) by the other. The computer that originates the TCP connection is usually the client, and the computer that accepts it is usually the server. Sometimes, notably in the FTP protocol, a secondary TCP connection will be established in the reverse direction, from the server to the client. But, in protocols other than FTP, connections are almost always initiated by the client.
Regardless of the direction in which a TCP connection is established, data can always flow both ways. However, the direction of the TCP connection matters because it determines who the initiating party is, and is also used by network components to impose rules on whether a connection can be established.
This article summarized from this site: https://www.bitvise.com/how-the-net-works
