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NETWORK BASICS

Network A system of interconnected computers and computerized peripherals such as printers is called computer network. This interconnection among computers facilitates information sharing among them. Computers may connect to each other by either wired or wireless media. A computer network consists of a collection of computers, printers and other equipment that is connected together so that they can communicate with each other.  


Network application
A Network application is any application running on one host and provides a communication to another application running on a different host, the application may use an existing application layer protocols such as: HTTP(e.g. the Browser and web server), SMTP(e.g. the email-client). And may be the application does not use any existing protocols and depends on the socket programming to communicate to another application. So the web application is a type of the network applications. 
There are lots of advantages from build up a network, but the th…

IP ADDRESSING

subnetting maskAn IP address is an address used in order to uniquely identify a device on an IP network. The address is made up of 32 binary bits, which can be divisible into a network portion and host portion with the help of a subnet mask. The 32 binary bits are broken into four octets (1 octet = 8 bits). Each octet is converted to decimal and separated by a period (dot). For this reason, an IP address is said to be expressed in dotted decimal format (for example, 172.16.81.100). The value in each octet ranges from 0 to 255 decimal, or 00000000 - 11111111 binary.  
Here is how binary octets convert to decimal: The right most bit, or least significant bit, of an octet holds a value of 20. The bit just to the left of that holds a value of 21. This continues until the left-most bit, or most significant bit, which holds a value of 27. So if all binary bits are a one, the decimal equivalent would be 255 as shown here: 
    1 1 1 1 1 1 1 1   128 64 32 16 8 4 2 1 (128+64+32+16+8+4+2+1=255) 
Here is a sample octet conversion when not all of the bits are set to 1. 
  0 1 0 0 0 0 0 1   0 64 0 0 0 0 0 1 (0+64+0+0+0+0+0+1=65) 
And this is sample shows an IP address represented in both binary and decimal. 
        10.       1.      23.      19 (decimal)   00001010.00000001.00010111.00010011 (binary) 
IP ADDRESSINGThese octets are broken down to provide an addressing scheme that can accommodate large and small networks. There are five different classes of networks, A to E. This document focuses on addressing classes A to C, since classes D and E are reserved and discussion of them is beyond the scope of this document. 


Internet Protocol hierarchy contains several classes of IP Addresses to be used efficiently in various situations as per the requirement of hosts per network. Broadly, the IPv4 Addressing system is divided into five classes of IP Addresses. All the five classes are identified by the first octet of IP Address. 
The first octet referred here is the left most of all. The octets numbered as follows depicting dotted decimal notation of IP Address
The number of networks and the number of hosts per class can be derived by this formula: 

When calculating hosts' IP addresses, 2 IP addresses are decreased because they cannot be assigned to hosts, i.e. the first IP of a network is network number and the last IP is reserved for Broadcast IP. 
Class A Address 
The first bit of the first octet is always set to 0 (zero). Thus the first octet ranges from 1 – 127, i.e. 

Class A addresses only include IP starting from 1.x.x.x to 126.x.x.x only. The IP range 127.x.x.x is reserved for loop-back IP addresses. 
The default subnet mask for Class A IP address is 255.0.0.0 which implies that Class A addressing can have 126 networks (27-2) and 16777214 hosts (224-2). 
Class A IP address format is thus: 0NNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH 
Class B Address 
An IP address which belongs to class B has the first two bits in the first octet set to 10, i.e. 

Class B IP Addresses range from 128.0.x.x to 191.255.x.x. The default subnet mask for Class B is 255.255.x.x. 
Class B has 16384 (214) Network addresses and 65534 (216-2) Host addresses. 
Class B IP address format is: 10NNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH 
Class C Address 
The first octet of Class C IP address has its first 3 bits set to 110, that is: 

Class C IP addresses range from 192.0.0.x to 223.255.255.x. The default subnet mask for Class C is 255.255.255.x. 
Class C gives 2097152 (221) Network addresses and 254 (28-2) Host addresses. 
Class C IP address format is: 110NNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH 
Class D Address 
Very first four bits of the first octet in Class D IP addresses are set to 1110, giving a range of: 

Class D has IP address rage from 224.0.0.0 to 239.255.255.255. Class D is reserved for Multi-casting. In multi-casting data is not destined for a particular host, that is why there is no need to extract host address from the IP address, and Class D does not have any subnet mask. 



This IP Class is reserved for experimental purposes only for R&D or Study. IP addresses in this class ranges from 240.0.0.0 to 255.255.255.254. Like Class D, this class too is not equipped with any subnet mask. 


A public IP address is an IP address that can be accessed over the Internet. Like postal address used to deliver a postal mail to your home, a public IP address is the globally unique IP address assigned to a computing device Private IP address on the other hand is used to assign computers within your private space without letting them directly expose to the Internet. For example, if you have multiple computers within your home you may want to use private IP addresses to address each computer within your home. In this scenario, your router get the public IP address, and each of the computers, tablets and smartphones connected to your router (via wired or Wi-Fi) get a private IP address from your router via DHCP protocol. Internet Assigned Numbers Authority (IANA) is the organization responsible for registering IP address ranges to organizations and Internet Service Providers (ISPs). To allow organizations to freely assign private IP addresses, the Network Information Center (InterNIC) has reserved certain address blocks for private use. The following IP blocks are reserved for private IP addresses. 

A private IP address is the address space allocated by InterNIC to allow organizations to create their own private network. There are three IP blocks (1 class A, 1 class B and 1 class C) reserved for a private use. The computers, tablets and smartphones sitting behind your home, and the personal computers within an organizations are usually assigned private IP addresses. A network printer residing in your home is assigned a private address so that only your family can print to your local printer. When a computer is assigned a private IP address, the local devices sees this computer via its private IP address. However, the devices residing outside of your local network cannot directly communicate via the private IP address, but uses your router's public IP address to communicate. To allow direct access to a local device which is assigned a private IP address, a Network Address Translator (NAT) should be used. 
A computer on the Internet is identified by its IP address. In order to avoid address conflicts, IP addresses are publicly registered with the Network Information Centre (NIC). Computers on private TCP/IP LANs however do not need public addresses, since they do not need to be accessed by the public. For this reason, the NIC has reserved certain addresses that will never be registered publicly. These are known as private IP addresses, and are found in the following ranges: 

  • From 10.0.0.0 to 10.255.255.255 
  • From 172.16.0.0 to 172.31.255.255 
  • From 192.168.0.0 to 192.168.255.255 


Static IP addressing is for one customer on one IP address and Dynamic IP addressing assigns a different IP address each time the ISP customer logs on to their computer, but this is dependent upon the Internet Service Provider (ISP) because some ISP's only change the IP address as they deem it necessary. If you have Dynamic IP Addressing through your Website Host it means that you are sharing an IP Address with several other customers. If you are a beginner on the internet, an avid internet user, are entertaining the thought of starting your own website business, are a gamer, use VOIP or VPN there are several things you should know about IP Addressing. 


If you feel the need to always know what your IP address is then you need a Static IP address, because it is constant. Static IP addresses are more reliable for Voice over Internet Protocol (VOIP), more reliable to host a gaming website or to play X-Box, Play Station, use Virtual Private Network for secure access to files from your company network computer, etc. Static IP addresses are also great if you use your computer as a server, as it should give your file server faster file uploads and downloads. Another plus with Static IP's, when hosting a website you are not sharing your IP with another company who sends out a lot of E-mail SPAM and not only has their website been shut down but in turn gets your IP address blacklisted. 
In contrast a static IP address can become a security risk, because the address is always the same. Static IP's are easier to track for data mining companies. Static IP addressing is less cost effective than Dynamic IP Addressing. 


The biggest advantages of Dynamic IP Addressing are less security risk as the computer is assigned a new IP address each time the customer logs on, they are cost effective and there is automatic network configuration (the less human intervention with network configuration the better). Dynamic addressing is usually used by ISP's so that one IP address can be assigned to several users, however some ISP's use Sticky Dynamic IP Addressing and do not change the IP address very often. Dynamic IP Addressing can be used by families with several computers or by a small business owner who has a home office. The software that comes with a router allows for Dynamic Host Configuration Protocol (DHCP) setup and assigns each computer attached to the router an IP address automatically. 
In contrast, Dynamic IP addressing should not be used for VOIP, VPN, playing online games or game hosting because Dynamic IP addressing is less reliable then Static IP addressing and could cause the service to disconnect while you are on a VOIP, VPN or gaming. 


Sub-netting is a technique to break up a large network into smaller networks by sacrificing bit Host ID on the subnet mask to be used as a new Network ID.Each IP class is equipped with its own default subnet mask which bounds that IP class to have prefixed number of Networks and prefixed number of Hosts per network. Classful IP addressing does not provide any flexibility of having less number of Hosts per Network or more Networks per IP Class. When changing a number in the Network part of an IP address we will be in a different network from the previous address. For example, the IP address 11.0.0.1 belongs to class A and has a default subnet mask of 255.0.0.0; if we change the number in the first octet (a block of 8 bits, the first octet is the leftmost 8 bits) we will create a different network. For example, 12.0.0.1 is in a different network from 11.0.0.1. But if we change a number in the Host part, we are still in the same Network. For example, 11.1.0.1 is in the same network of 11.0.0.1. 


Let’s come back to our example with the IP 11.0.0.1, we will write all numbers in binary form to reveal what a computer really sees in an IP address. 
Now you can clearly see that the subnet mask will decide which is the Network part, which is the Host part. By borrowing 8 bits, our subnet mask will be like this: After changing the second octet of the subnet mask from all “0” to all “1”, the Network part is now extended. Now we can create new networks by changing number in the first or second octet. This greatly increases the number of networks we can create. With this new subnet mask, IP 11.1.0.1 is in different network from IP 11.0.0.1 because “1” in the second octet now belongs to the Network part. So, in conclusion we “subnet” by borrowing bit “0” in the Host portion and converting them to bit “1”. The number of borrowed bits is depended on how many networks we need. 

Note: A rule of borrowing bits is we can only borrow bit 0 from the left to the right without skipping any bit 0. For example, you can borrow like this: “1111 1111. 1100 0000.0000 0000.0000 0000” but not this: “1111 1111. 1010 0000.0000 0000.0000 0000”. In general, just make sure all your bit “1”s are successive on the left and all your bit “0”s are successive on the right. 


Because we can change any bit in the second octet to create a new subnet, each bit can be “0” or “1” so with this subnet mask (255.255.0.0) we can create 28 more subnets. From here we can deduce the formula to calculate the newly created subnets. Suppose n is the number of bits we borrow: 
The number of newly created subnets = 2n 
In our example, we borrow 8 bits so we will have 2n = 28 = 256 subnets! 


The number of hosts per subnet is depended on the Host part, which is indicated by the “0” part of the subnet mask. So suppose k is the number of bits “0” in the subnet mask. The formula to calculate the number of hosts is 2k. But notice that with each subnet, there are two addresses we can’t assign for hosts because they are used for network address & broadcast address. Thus we must subtract the result to 2. Therefore the formula should be: 
The number of hosts per subnet = 2k – 2 
In our example, the number of bit “0” in the subnet mask 255.255.0.0 (in binary form) is 16 so we will have 2k – 2 = 216 – 2 = 65534 hosts-per-subnet! 
Sub-netting Example A company has just been assigned the network 198.23.16.0/28. How many subnets and hosts-per-subnet you can create with a subnet mask of 255.255.255.252? 
Solution Company was given a “sub-netted” network from the beginning and it is not using the default subnet mask. So we will compare two subnet masks above: 
/28 = 1111 1111.1111 1111.1111 1111.1111 0000 (=255.255.255.240) 255.255.255.252 = 1111 1111.1111 1111.1111 1111.1111 1100 (= /30) 

In this case we borrowed 2 bits. Therefore: 

The number of newly created subnets = 22 = 4 (with 2 is the borrowed bits) The number of hosts per subnet = 22 – 2 = 2 (with 2 is the bit “0”s left in the 255.255.255.252 subnet mask) 
In this exercise I want to go a bit deeper into the subnets created. We learned there are 4 created subnets but what are they? To find out, we should write all things in binary: Because two subnet masks (/28 & /30) only affect the 4th octet so we don’t care about the first three octets. In the 4th octet we are allowed to change 2 bits (in the green box) of the IP address to create a new subnet. So there are 4 values we can use: 00, 01, 10 & 11. After changing, we convert them back to decimal numbers. We get 4 subnets: + First subnet: 198.23.16.0/30 (the 4th octet is 00000000) + Second subnet: 198.23.16.4/30 (the 4th octet is 00000100) + Third subnet: 198.23.16.8/30 (the 4th octet is 00001000) + Fourth subnet: 198.23.16.12/30 (the 4th octet is 00001100) 
So how about hosts per subnet? Please notice that all these 4 subnets are successive. So we can deduce the range of these subnets: + First subnet: ranges from 198.23.16.0 to 198.23.16.3 + Second subnet: ranges from 198.23.16.4 to 198.23.16.7 + Third subnet: ranges from 198.23.16.8 to 198.23.16.11 + Fourth subnet: ranges from 198.23.16.12 to 198.23.16.15 



A. 191.192.168.1  
B. 191.168.169.254  
C. 172.32.255.0  
D. 172.31.12.251 


A. 191.10.0.1 255.0.0.0  
B. 127.10.0.1 255.0.0.0  
C. 128.10.0.1 255.0.0.0  
D. 126.10.0.1 255.0.0.0 


A. 10.1.1.1 255.255.0.0  
B. 126.1.1.1 255.255.0.0  
C. 129.1.1.1 255.255.0.0  
D. 192.168.1.1 255.255.0.0 


A. 8 bits  
B. 16 bits  
C. 24 bits  
D. 32 bits 


A. 64  
B. 128  
C. 192  
D. 255 


A. ping 127.0.0.1  
B. ping 0.0.0.0  
C. ping: 1  
D. trace 0.0.: 1 

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