What is a Network ID?
What is an Host ID?
What does a subnet mask do?
Which layer of TCP/IP Model "routes" the packet?
An IP (Internet Protocol) address uniquely identifies a node or host connection to an IP network. System Administrators or network designers assign IP address to hosts (nodes). IP addresses are configured by software, not hardware specific. An IP address consist of two parts, Network address and Node (host) address.
decimal=binary
192 = 11000000
| 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
| 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 |
TCP/IP Address Classes --First Octet
| Class | Start Binary | Finish Binary | Start Decimal | Finish Decimal | Networks | Host per Network |
| A | 00000001 | 011111111 | 1 | 127** | 126 | 16,777,214 |
| B | 10000000 | 10111111 | 128 | 191 | 16,384 | 65,534 |
| C | 11000000 | 11011111 | 192 | 223 | 2,097,152 | 254 |
Note: the first byte of the first octet determines the class of the address.
**(127 reserved for loopback and internal testing on the local machine)
| Class | Start Binary | Finish Binary |
| A | 00000001 | 011111111 |
| B | 10000000 | 10111111 |
| C | 11000000 | 11011111 |
Class A networks start with 0
Class B networks start with 10
Class C networks start with 110
Default Subnet Mask
Class A = 255.0.0.0
(11111111.00000000.00000000.00000000)
Class B = 255.255.0.0 (11111111.11111111.00000000.00000000)
Class C = 255.255.255.0 (11111111.11111111.11111111.00000000)
Broadcast = 255.255.255.255 (11111111.11111111.11111111..11111111)
| 220 | 219 | 218 | 217 | 216 | 215 | 214 | 213 | 212 | 211 | 210 | 29 | 28 | 27 | 26 | 25 | 24 | 23 | 22 | 21 | 20 |
| 1,048,576 | 524,288 | 262,144 | 131,072 | 65,536 | 32,768 | 16384 | 8192 | 4096 | 2048 | 1024 | 512 | 256 | 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
Subnetting is an IP addressing scheme used for subnets. It enables you to take a single IP address from your ISP and make a group of networks out of it. You can then route between these networks internally and through your main router externally. It is a physical segment in a TCP/IP environment that uses IP addresses from a single network ID. The number of bits in the subnet mask will determine the number of subnets and hosts per subnets available to you.
IP layer uses the subnet mask to determine whether a host is on the local network or a remote network. To do this the bits of the subnet mask are turned on (255) for the portion that represents the network ID. The ANDing process pulls the first 16 bits from the IP address, which is the network ID. (an example is on page 95, figure 6.1) NOTE: there is a correction to the subnet mask in Table 6.1, it should be 255.255.0.0 rather than 255.255.255.0
What happens when a network is "subnetted" . You are simply set two or more extra bits to the "on" in the subnet mask. This way, IP layer sees more of the hosts with which you are communicating as being on a remote network (see Table 6.2)
Extra bits are used to identify the network
128 = 10000000 = 1 bit
192 = 11000000 = 2 bits
224 = 11100000 = 3 bits
240 = 11110000 = 4 bits
248 = 11111000 = 5 bits
252 = 11111100 = 6 bits
254 = 11111110 = 7 bits
255 = 11111111 = 8 bits
Quick Way to Determine a Number of Subnets and the number of Hosts per subnet.
Subnet = 2N-2
Host = 2N
Class C example
Let's say you've got a subnet of 255.255.255.240
24 - 2 = 16 - 2 = 14 (240 Class C Subnet has 14
Networks)
24-2 = 16 - 2 = 14 (240 Class C Subnet has 14 host per
subnet)
Class B example
Let's say you've got a subnet of 255.255.240.0
24 - 2 = 16 - 2 = 14 (240 Class B Subnet has 14
Networks)
212 - 2 = 4096 -2 = 4094 (240 Class C Subnet has 4096
host per subnet)
Class A example
Let's say you've got a subnet of 255.240.0.0
24 - 2 = 16 - 2 = 14 (240 Class A Subnet has 14
Networks)
220 - 2 = 1048576 -2 = 1048574 (240 Class A Subnet has
1048574 host per subnet)
Let's take at look at Table 6.3 on page 96.
| IP Address | 1st Octet | 2nd Octet | 3rd Octet | 4th Octet |
| 160.16.154.23 | 10100000 | 00010000 | 10011010 | 00010111 |
| 255.255.0.0 | 11111111 | 11111111 | 00000000 | 00000000 |
| 160.16.0.0 | 10100000 | 00010000 | 00000000 | 00000000 |
| 255.255.240.0 | 11111111 | 11111111 | 11110000 | 00000000 |
| 160.16.144.0 | 10100000 | 00010000 | 10010000 | 00000000 |
IP Address of 160.16.154.23 with the default subnet mask (255.255.0.0)= 160.16.0.0
IP Address of 160.16.154.23 with a 240 subnet mask = 160.16.144.0
Example 1
Is host 131.107.32.1 on the same subnet as 131.107.63.254 using a subnet mask of 255.255.224.0?
| IP address | 131.107.32.1 | 10000011.01100001.00100000.00000001 |
| Subnet Mask | 255.255.224.0 | 11111111.11111111.11100000.00000000 |
| Network ID | 131.107.32.0 | 10000011.01100001.00100000.00000000 |
| IP address | 131.107.63.254 | 10000011.01100001.00111111.11111110 |
| Subnet Mask | 255.255.224.0 | 11111111.11111111.11100000.00000000 |
| Network ID | 131.107.32.0 | 10000011.01100001.00100000.0000000 |
Answer is YES.
Let's look at the third octet of IP address and Subnet mask
| Host 3rd Octet | 32 | 00100000 |
| Subnet 3rd Octet | 224 | 11100000 |
| Network ID 3rd Octet | 32 | 00100000 |
| Host 3rd Octet | 63 | 00111111 |
| Subnet 3rd Octet | 224 | 11100000 |
| Network ID 3rd Octet | 32 | 00100000 |
If you compare the third octet of both addresses you'll see that the first IP address is on network 131.107.32.0 and the second IP address is also on network 131.107.32.0, these two host are local to one another.
Example 2
Is host 131.107.32.1 on the same subnet as 131.107.64.254 using a subnet mask of 255.255.224.0
| IP address | 131.107.32.1 | 10000011.01100001.00100000.00000001 |
| Subnet Mask | 255.255.224.0 | 11111111.11111111.11100000.00000000 |
| Network ID | 131.107.32.0 | 10000011.01100001.00100000.00000000 |
| IP address | 131.107.64.254 | 10000011.01100001.01000000.11111110 |
| Subnet Mask | 255.255.224.0 | 11111111.11111111.11100000.00000000 |
| Network ID | 131.107.64.0 | 10000011.01100001.01000000.0000000 |
Answer is NO, here's why
Let's look at the third octet of IP address and Subnet mask
| Host 3rd Octet | 32 | 00100000 |
| Subnet 3rd Octet | 224 | 11100000 |
| Network ID 3rd Octet | 32 | 00100000 |
| Host 3rd Octet | 64 | 01000000 |
| Subnet 3rd Octet | 224 | 11100000 |
| Network ID 3rd Octet | 64 | 01000000 |
If you compare the third octet of both addresses you'll see that the first IP address is on network 131.107.32.0 and the second IP address is on network 131.107.64.0. These two host are remote to one another, you will need a router
The first two octets of subnet mask are the same hence 160.16. Now lets look at the 3rd octet. With the first subnet mask (default one) the only bits that are turned on are the first 16 bytes (Network ID).