What is a IP address?

Ensures the proper delivery of data (packet data) to each PC on the network (for PC networks forming part of the label is often used by guest). If there is a computer on which the user is connected, a different network, IP protocol route packets through the computer to the network interconnect (ie. The goal).

An IP 192.168.ll cooperate closely other protocols, for example. ARP (Address Resolution Protocol) is used to map IP addresses, sometimes referred to as logical addresses to physical addresses (hardware addresses of network adapters). We must realize that if we enter the address of the PC, you enter the logical network address (IP address).

By the hardware when the packet is moved to the lead in the form of electrical signals are evaluated, however, the physical address of the adapter that are e.g. in the case of fixed Ethernet adapter manufacturer already set.

Protocol then determined by us entering the IP address of the physical address of the adapter. TCP (Transmission Control Protocol) establishes a connection between computers on the network. If we IP address within the network to establish a connection with any of the computers, we must know the identifier that distinguishes it from others. Computers on the Internet is identified by a number called an IP address.

This ID is unique, that is, within the Earth no two computers with the same IP address. When we have to be more precise IP address does not apply to the computer, but its AC adapter. In fact, there are internet computers that contain multiple network adapters, each of which has its own IP address.

More Adapter also include gateways (gateways) – There are special computers that connect LANs together and ensure proper routing data between networks. IP address consists of 4 bytes. Most often writes so-called dotted notation when each byte of the address expressed by decimal number and these numbers are separated by dots.

Address can then look like this: To create your own network and does not envisage its interconnections with other networks, define the IP addresses themselves. We take care to be on the network repeated. We counted with a connection to an existing network such as the Internet, we have the assignment of IP addresses to request the authority entrusted with managing the entire network. Only because she has an overview of the already allocated and that still fall well as free IP address.

Because the computers are located in the Internet are connected to interconnected local networks, the problem of data transfer between PC problem of data transfer between networks. Gateway aimed data between networks, working with so-called network address. The network address is nothing more than a certain proportion of IP addresses. The remaining part of the IP address can then address the PC within the network. 192nd 168th 30th 15 Network address (network part of the address) address of the PC in the network Thus, we see that the IP address is divided into components representing a network address portion representing the address of the PC in the network. Depending on how large the individual networks (many PC content), IP addresses are divided into three main classes.

Classes vary the number of bits reserved for the network and the PC part of the address. From the number of bits reserved for each part of the IP address can easily determine how many total networks can exist, for example, Type B and how each network the computer contents.

There are several addresses that may not be used as addresses of computers on the network:
1) and called loopback address. This address uses a network software for testing purposes. If we send data to this address and will not be transmitted via any of the network adapters in the PC network. So we can determine whether our software is functional regardless of whether it works network hardware;

2) Network addresses, ie. the address, computer part containing all zeros. These addresses are used to correct IP protocol to route packets between networks;

3) Broadcast addresses, which often contain the same unit. They are used to the broadcast packets. If we send a packet to the broadcast address, so it will get all the computers on the network. Net mask number is 4 bytes long, which determines which portion of an IP address reserved for the network address. Position containing the binary is to identify the network part of the address and position containing binary zeros determine the computer part of the address.

For a network of C thus we enter the mask Symbolic names In the previous section we have become familiar with IP addresses. We see that are 32-bit numbers. Because the handling of longer numbers for people boring and impractical, was created a mechanism that allows the user to enter IP addresses of computers instead of their symbolic names. Operating software then performs the mapping of symbolic names to IP addresses automatically. Unless otherwise provided, the symbolic names are mapped to IP addresses in accordance with a set of hosts.

This file contains IP addresses 192.168.ll and symbolic names in the following format:
IP_adresameno_počítača In any case should contain the loopback address and the address of the computer on which it is located.
Furthermore, of course, IP addresses and symbolic names of computers with which we communicate.

Hosts file may then look like this:
192.168.ll localhost loopback Lucy xxx903 f m Sipok mirec 1192.168.ll IP addresses to symbolic names separated by a space. Localhost is the local computer on which it resides, and Lucy and mirec are the names of computers on the Internet communicate with. Note the more symbolic names listed on a line separated by spaces. They are called. aliases, which may be on one computer are a few.

All can be used in the process of establishing the connection. It is clear that in the hosts file must be given symbolic names and IP addresses of the PCs to which we want to access by the name. In small networks, it is no problem to provide. However, consider a network such as the Internet, containing several million PCs. If we want to have access to all this PC, we need to have on your computer in the hosts file several million rows.

The file size is not the only problem. Obviously, this scale network changes dynamically, constantly being updated with new nodes (servers) and some are canceled. How then to ensure adequate replenishment of the hosts file on our (and not only in our) PC? Because of this, in networks based on the TCP / IP system realized so. domain names and system enabling to obtain from these domain names IP addresses. All Internet nodes are then broken down so-called. domains, forming a hierarchical structure.

On top of the structure is the main root node, followed by the domain indicating each State Party to the Internet. E.g. sk domain name is reserved for the Slovak Republic, CZ for the Czech Republic, AT Austria, DE, UK United Kingdom, etc.. In the US, the birthplace of the Internet is the same as the domain of the individual countries of the world use the domain reflecting the type of nodes associated to them (edu for educational establishments, commercial organizations com, gov government, army mil and org other non-commercial organizations).

It continues structure of second-level domain, which builds additional levels already largely based on local networks of the organization. The sense of this hierarchical structure is that the accuracy and timeliness of information about domain names and IP address does not match one central node, but this information is distributed to each domain and IP address information providing nodes in domains reserved for this activity. This service (mapping domain names to IP addresses) is known as DNS nodes, which does so called nameservers.

For each domain, thus there nameserver (actually should be because of the reliability of the entire system at least two) who knows all the names and IP addresses of computers in your domain name server address plus the superior domain. Then we use on our PC domain name, it is automatically mailed query on the resolution of the (mapped to the corresponding IP address) of the next higher name server.

If the name server can not resolve the name, if it occurs if it is not in the domain name server sends a query supervisor who attempts by the resolution of. If that fails even to him and is forwarding the query to other superior or subordinate domains. In practice, the resolved names kept for some time in the cache nameserver, which are then able to request another name saved comply immediately, without having to refer to other nameservers.

E-mail you are about everyone he met this definition and can not imagine what benefits e-mail, also referred to as e-mail is. First of all, it is the speed at which they are delivered to our letters. If we are connected to the Internet, the time of receipt of the letter, for example. overseas, compared to snail mail negligible. The cost of sending a letter to be smaller. The process itself to send and receive the letter is much easier than with postal mail, by saving paper and places not to mention.

Sometimes the e-mail as the preferred phone. We do not get upset if they can not reach the called party all day and it just our requests or questions send e-mail. He reads the person immediately upon arrival at their workplace. There is also a problem to send a letter to multiple recipients at once. We may not make copies of the letter on the copier, but just enter the command to be sent a letter to several persons, and the rest will take care system itself. E-mail is not limited only to send and receive letters. Widely used in the so-called. conferences (newsgroups), through which anyone can take part in discussions on the right to conferences previously learned.

Letter sent by e-mail does not contain only text messages, but also any file, so you may send and programs, pictures, audio files etc. E-mail has the further advantage that its scope is not limited to the network that have a direct connection, but we can also send letters to other networks if they are connected to our network through a gateway. If we want to send mail to remote users (eg. Internet) we must specify the full email address of the user. Such an address consists of a user name and domain name for the computer where the user is working.

The name is the domain name separated computers @ sign. eg .:
lucy@markiza.sk approach [1] on TCP / IP networks 192.168.ll now we discuss basic concepts of the Internet, the packet routing network. Knowledge of these issues will help us overcome any problems arising from network communications.

Routing (routing) is the process of finding the path that must be sent packets to arrive to the destination node network. A computer that decides the direction of transmission of packets is called a gateway (gateway), or even a router (router). There are two types of routing, through direct and indirect routing. Direct – packets are transmitted from the source PC directly to the target computer via a transmission medium. This is about the case where the two PCs are on the same “cable” network.

Indirect smerovanie- packets are transmitted to a network that is not directly connected to that computer. Packet delivery procure goal. This occurs if there are more interconnected networks (internet).

Here are the different steps in the process of direct routing:
1) The IP address of the target machine is found to have a physical address,

2) The packet is sent via a transmission medium to the identified physical address of the target PC. Here again is described a process of indirect routing that

can not take place when the packet is sent directly to the target computer:
1) The packet is sent directly to the nearest gate.

2) If the gate does not lie in the network, where it is going, our package, select the routing gateway software more directly reachable gateway situated on the way to your destination.

3) Finally the packet reaches the gateway that can be sent directly to the computer for which it is intended. In order to defend the right to transmit packets must know the network topology. That is, it must know the nearest goal through which to send the packet arrived at the destination network.

Let us describe the algorithm used by the gateway when deciding where to send the packet delivery address:
1) Find the IP address 192.168.ll of the packet destination computer.

2) the IP address determines the network to which the packet direction (network part of the address)

3) If the network address corresponds to the one directly connected to the gateway network, check your physical address PC

4) If the network address is not directly connected network address, but is found in the routing table and sends the packet to the information given in the routing table.

5) but if the network address is not directly connected network address, or the address listed in the routing table, routing signals an error. We see that the gateway software used for routing packets to indirectly connected networks, so-called routing table (routing table).

We will show you the format: network address of the gateway IP address
ddress X direct service network IP address of the network gateway IP address Y a default gateway IP address B left part of the table compares the gateway software with network part of the IP address obtained from the packet. If the network address of the packet matches the network address from the table, the packet is sent to the gateway IP address from the right side of the table (or the direct service).

Note the Default entry for the local network addresses. If the routing table row with the item and all packets for which it was not found in the table corresponding to the network address, sent to the gate of that for DEFAULT. In our case, the gate B. The above-described mechanism must be implemented not only at the goal, but partly also on all computers in the network that have to communicate with computers on other networks.

When we used, for example. workstation and want to be with her, with Telnet, connect to your computer from another network station software must know what gate has over Telnet packets sent. It must therefore include the above routing table. How to connect? Now we explain the connection to the network, based on a set of TCP / IP.

First, we make a list of items that we allow the network connection, and then stopped at each one.
1) Network hardware

2) undertaking the software on our computer file TCP / IP

3) address of your computer on the network

4) address of the computer with which we communicate network hardware. Choosing the right network hardware that you miss in your PC depends on the connection point through which we will be connected to the network.

If TCP / IP will be connecting through a network adapter. We use the AC adapter to connect to the local network to communicating through TCP / IP. Depending on the type of network media select the appropriate adapter type. We may then need an adapter for Ethernet, Token Ring, twisted pair (twisted pair), fiber optic cable, etc. The AC adapter may be implemented as an expansion card, which retracts into a free slot of the PC or in an external device connectable to a USB port, or as a PCMCIA adapter. The software that realizes TCP / IP.

The implementation of TCP / IP is now available for all major operating systems. Any computer user should have the possibility to make use of TCP / IP. When selecting a network adapter we need to think about the appropriate drivers for the adapter that will support the operating system. After installing the system has yet to be set for the use of our network, listing our network address and subnet mask, make some devices, and so on. Address of the computer on the network. Each PC must have a network in your IP address.

In order to be connected to any of the international network, we have to apply for a network address. Assigned network part of the IP address is mandatory for us (it can not be changed). The computer part of the IP address of each network interface define. However, we must pay attention to the unique IP address thus obtained (due to lack of addresses A and B of the Internet, it is the last time that build an extensive network of candidates allocated a number of addresses of type C). If you build your own, isolated network and computer network can share IP addresses define. The network part of the address is defined by the expected extent of our network.

Depending on the number of computers on the network and the number of networks in our internet we determine the network type (A, B or C). Network type, and can contain more than 16 million (216) computers and the networks can be built on the Internet 128 (27). IN 16384 (214) B networks can be 65 536 (216) Computers and networks Type C can be created over 2 million and each can contain 256 (28) computers.

From the above values we do not forget to deduct the reserved address. If we defined the type of network it can be determined at its discretion, the numerical value of the IP network (pay attention to the first bits identifying the type of network) then further define the computer part of the address for each PC on the network. Address of the computer with which we communicate.

Unless you are using Domain Name System (DNS), we need to define the computer’s hosts file containing mapping Using symbolic names to IP addresses. The corresponding IP address of your network administrator, we want to communicate. What is a firewall Firewall is a special machine, which is the only link between the outside world (Internet) and our network, we want to protect from external influences.

This PC does not give the outside world the routing information (routing) in our network and causes so that our network invisible to the outside world.

If we want-configurable firewall, we need to remember the following:
1) The firewall does not promote (not provide) routing information. That is, if one wants to join our network from a computer that is on the external network, it must first login to the firewall, and from it to any other computer in the external network. Similarly, if someone wants to attach the external network connection to the computer from our internal network must log on to the firewall (there must have an account) and up from it can go to a specific machine’s internal network;

2) any e-mail sent by users of the internal network to external networks must be routed to the firewall. Firewall must also receive all incoming mail from the external network and distribute it to users of the internal network. This may be ensured e.g. defining an alias for each user;

3) firewall must in no way to interconnect via NFS file systems, nor any files and directories provide;

4) on the firewall must be the establishment of maximum discipline in maintaining access passwords; It goes without saying firewall must live under constant and careful control. Through relevant programs must follow all its activities and in case of emergency immediate action.

Because if any unauthorized penetration of the firewall is the way to open our internal network. Currently there are many programs that can facilitate the secure firewall, including a variety of traps, used for detection and subsequent disposal of uninvited intruder.



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