2. Как работи Интернет? Основи на TCP/IP мрежите.

- 7-слоен OSI модел на компютърните мрежи;
    The Open Systems Interconnection Reference Model (OSI Model or OSI Reference Model for short) is a layered abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnect initiative. It is also called the OSI seven layer model.
    The following diagram attempts to show where various TCP/IP and other protocols would reside in the original OSI model:
e.g. HTTP, SMTP, SNMP, FTP, Telnet, NFS
e.g. XDR, ASN.1, SMB, AFP
e.g. ISO 8327 / CCITT X.225, RPC, NetBIOS, ASP
Data Link
e.g. Ethernet, Token ring, PPP, HDLC, Frame relay, ISDN, ATM
e.g. electricity, radio, laser

- пакет от протоколи TCP/IP
    The Internet Protocol (IP) is a data-oriented protocol used by source and destination hosts for communicating data across a packet-switched internetwork.
    Transmission Control Protocol (TCP) is a connection-oriented, reliable delivery byte-stream transport layer protocol
currently documented in IETF RFC 793 [1] (http://www.ietf.org/rfc/rfc793.txt).
    The User Datagram Protocol (UDP) is a minimal message-oriented transport layer protocol that is currently documented in IETF RFC 768.
    The Internet Control Message Protocol (ICMP) is part of the Internet protocol suite and defined in RFC 792. ICMP messages are typically generated in response to errors in IP datagrams (as specified in RFC1122 (http://www.ietf.org/rfc/rfc1122.txt)) or for diagnostic or routing purposes.
    The Internet Group Management Protocol (IGMP) is a communications protocol used to manage the membership of Internet Protocol multicast groups. IGMP is used by IP hosts and adjacent multicast routers to establish multicast group

- 4-слоен модел на TCP/IP мрежите:
Application приложно ниво
Transport транспортно ниво
Internet Интернет ниво
Link свързващо ниво

- IP адреси:
    Адресите на възлите (компютрите) в Интернет (IP-номерата) са 32-битови числа, обикновено записвани като 4 десетични числа (октети или байта), разделени с точки, например
    Използват се 3 основни типа (класа) адреси, което е направено с цел да се получи добро решение на проблема всеки адрес да указва както мрежата, така и компютъра в нея. Основното предположение е, че има много на брой мрежи, но повечето от тях са малки. Приема се, че 24 бита (224=16777216) са достатъчни за номерирането на всички мрежи в света. Въведени са 3 типа адреси, което води и до 3 класа мрежи.
    -- клас A:  1 байт за адрес на мрежата (числата от 1 до 126) + 3 байта за номериране на компютрите в мрежата; броят на всички мрежи от този клас е до 126.
    -- клас B:  2 байта за адрес на мрежата (от 128.1 до 191.254, числото 127 в първия байт, както и числата 0 и 255 във втория, са запазени за специални цели) + 2 байта за адресиране на компютрите в мрежата (64516 броя); броят на всички мрежи от този клас е до 16256.
    -- клас C: 3 байта за адрес на мрежата (от 192.1.1 до 223.254.254) + 1 байт за номериране на компютрите в мрежата (254 броя); броят на всички мрежи от този клас е до 2064512.
    Адресите, започващи с число над 223, образуват класовете D и E и са запазени за специални цели.

    IP addresses are not easy to remember, even using dotted decimal notation. The Internet has adopted a mechanism, referred to as the Domain Name System (DNS), whereby computer names can be associated with IP addresses. These computer names are referred to as domain names. The DNS has several rules that determine how domain names are constructed and how they relate to one another. For the purposes of this chapter, it is sufficient to know that domain names are computer names and that they are mapped to IP addresses.

- протоколите TCP и UDP
    TCP (Transmission Control Protocol) is a connection-based protocol that provides a reliable flow of data between two computers.
   UDP (User Datagram Protocol) is a protocol that sends independent packets of data, called datagrams, from one computer to another with no guarantees about arrival. UDP is not connection-based like TCP.
    The TCP and UDP protocols use ports to map incoming data to a particular process running on a computer. In datagram-based communication such as UDP, the datagram packet contains the port number of its destination and UDP routes the packet to the appropriate application, as illustrated in this figure:

- портове - какво представляват и защо са необходими?
    Port numbers range from 0 to 65,535 because ports are represented by 16-bit numbers. The port numbers ranging from 0-1023 are restricted; they are reserved for use by well-known services such as HTTP and FTP and other system services. These ports are called well-known ports. Your applications should not attempt to bind to them.
source IP;
source port;
destination IP;
destination port;

- сокети
    Сокет наричаме двойката (IP адрес, номер на порт), комуникационен канал за една TCP връзка.

- как работят сокетите и портовете?
IE = <-> = Appache Web Server
IE = <-> = Appache Web Server
OE= <-> = POP3 Server
FTP= <-> = FTP Server

- сървери и клиенти
Клиент - свързва се към сървера и отваря сокет.
Сървер - "слушат на определен порт" и чакат клиетско приложение да се свърже към тях.

- програмен модел сървер-клиент
Магазин с няколко щанда за клиенти.

- портовете при UDP протокола
Няма комуникационен канал, а се изпращат и получават единични пакети.

- протоколи
Формалните езици, които се използват за комуникация в компютърните мрежи се наричат протоколи.

- услуги в Интернет и стандартни номера на портове
    Popular Internet application protocols are associated with well-known ports and wel-known Internet services. The server programs implementing these protocols listen on these ports for service requests. The well-known ports for some common Internet application protocols are:
Port Protocol Service description
21 File Transfer Protocol(FTP) Transfers files
22 >SSH (Secure Shell Protocol) Allows secure remote administration through standard shell (console)
25 Simple Mail Transfer Protocol Send email
80 HyperText Transfer Protocol(HTTP) Accesse the WWW (World Wide Web)
110 Post Office Protocol Receive email

The well-known ports are used to standardize the location of Internet services.

- IPv6 протокол
      IPv6 is version 6 of the Internet Protocol. IPv6 is intended to replace the previous standard, IPv4, which only supports up to about 4 billion (4x109) addresses, whereas IPv6 supports up to about 3.4x1038 addresses.
     IPv6 is the second version of the Internet Protocol to be widely deployed, and is expected (as of 2001) to form the basis for future expansion of the Internet. In 2003, Nihon Keizai Shimbun (as cited in CNET Asia Staff, 2003) reported that Japan, China, and South Korea claimed to have made themselves determined to become the leading nations in internet technology, which would partially take the form of jointly developing IPv6, and completely adopting IPv6 starting in 2005.
     The compelling reason behind the formation of IPv6 was lack of address space, especially in the heavily populated countries of Asia such as India and China.
     IPv6 addressing
     The most dramatic change from IPv4 to IPv6 is the length of network addresses. IPv6 addresses, as defined by RFC 2373 and RFC 2374, are 128 bits long; this corresponds to 32 hexadecimal digits, which are normally used when writing IPv6 addresses, as described in the following section.
    The number of possible addresses in IPv6 is 2128. The number of IPv6 addresses can also be thought of as 1632 as each of the 32 hexadecimal digits can take 16 values (see combinatorics).  In some situations, IPv6 addresses are composed of two logical parts: a 64-bit network prefix, and a 64-bit host-addressing part, which is often automatically generated from the interface MAC address.
     Notation for IPv6 addresses
     IPv6 addresses are 128 bits long but are normally written as eight groups of 4 hexadecimal digits each. For example,
 is a valid IPv6 address.  If a 4 digit group is 0000, it may be omitted. For example,
 is the same IPv6 address as
     Following this rule, if more than two consecutive colons result from this omission, they may be reduced to two colons, as long as there is only one group of more than two consecutive colons. Thus
 are all valid and mean the same thing, but
 is invalid.  Also leading zeros in all groups can be omitted, thus
 is the same thing as
     IPv4 addresses are easily convertible to IPv6 format. For instance, if the IPv4 address was,
it could be converted to
Then again, one could use the hybrid notation (IPv4 mapped addresses), in which case the address would be