IP Addresses, Subnet Masks, & Subnetting
An IP address is used for Network Layer identification of hosts and routers on a TCP/IP network. The address consists of a 32-bit binary number of 4 octets and is usually displayed in the decimal format 100.100.100.100, which is called dotted decimal notation.
The class of the Network is determined by the high order bits
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Bitwise ANDing
Hosts and routers use Boolean math to determine the netid and the hostid by the use of ANDing.
Bit 1 | Bit 2 | Result |
1 | 1 | 1 |
1 | 0 | 0 |
0 | 1 | 0 |
0 | 0 | 0 |
ANDing IP Addresses and Subnet Masks
198.53.147.45 | 11000110 | 00110101 | 10010011 | 00101101 |
255.255.255.0 | 11111111 | 11111111 | 11111111 | 00000000 |
Result | 11000110 | 00110101 | 10010011 | 00000000 |
Note: In all calculations and comments the current classful standard is used. Under this standard, the first and last subnets are dropped.
The higher order bits of 198.53.147.45 are 110 thus this is a class C address
The Netid of this Class C address 198.53.147.0 and the Hostid is 45
From the example above it is visible that with this subnet mask the netid and hostid remained unchanged by bitwise ANDing.
Defining a Subnet Mask
Subnetting is a technique that allows the network administrator to divide a network into smaller networks by using the same network number assignment. The advantages of subnetting are below:
In the diagram above illustrates how a subnetted IP appears on an internal Intra-Net and the Internet. The Internet only reads the NetId and the routers on the Internet are only concerned with routing the IP packet to the Intra-Net external router. When the IP packet reaches the external router, which has been configured for subnet routing reads the HostId. The router then forwards the packet to appropriate subnet where it is delivered to the host.
Determining the Subnet Mask to Use
The diagram below shows that when subnetting an IP address the Netid remains unchanged but the Hostid is further sectioned or divided up.
The Current Classfull Standard
There a number of different ways to define the available range of IP addresses. The MCSE exams use the existing Classful standard. Some network equipment may follow a more current addressing scheme called CIDR. To date, CIDR has not been adopted as the standard."
Just for reference, MS stacks do not adhere to the Classful standard. They will allow you to use all subnets, including the first and last. For example, look at the KB article, Q139983 "Class C Subnetting Options for RAS Routing"
The Current Classfull Standard rules are as follows.
Before defining a subnet mask, you need to determine the number of segments and hosts per segment
Bit Pattern | Masked Bits | Provided Subnets | Subnet Mask |
11000000 | 2 | 2 | 192 |
11100000 | 3 | 6 | 224 |
11110000 | 4 | 14 | 240 |
11111000 | 5 | 30 | 248 |
11111100 | 6 | 62 | 252 |
11111110 | 7 | 126 | 254 |
11111111 | 8 | 254 | 255 |
The above chart represents how a subnet mask is represented in binary format, to determine the values is quite simple
Question: What happened to 198.53.147.32 and 198.53.147.63 why can't these be IP addresses ?
198.53.147.32 - Is the subnet's
Network Id which is revserved for the physical segment
(00100000).
198.53.147.63 - Is the subnet's Broadcast Address
(00111111)
As stated above every network or subnet has a IP address for the Physical segment and a Broadcast Address.
Calculating Hosts
Bit
Pattern |
Masked
Bits |
Provided
Subnets |
Subnet
Mask |
Class A
Hosts/Subnet |
Class B
Hosts/Subnet |
Class C
Hosts/Subnet |
11000000 | 2 | 2 | 192 | 4,194,301 | 16,382 | 62 |
11100000 | 3 | 6 | 224 | 2,097,150 | 8190 | 30 |
11110000 | 4 | 14 | 240 | 1,048,574 | 4,094 | 14 |
11111000 | 5 | 30 | 248 | 524,286 | 2,046 | 6 |
11111100 | 6 | 62 | 252 | 262,142 | 1,022 | 2 |
11111110 | 7 | 126 | 254 | 131,070 | 510 | 0 |
11111111 | 8 | 254 | 255 | 65,534 | 254 | 0 |
Question: I want to set up a TCP/IP LAN in my organization what IP addresses should I use?
For hosts that need to be connected to the Internet you can get a block of valid IP addresses from a ISP. Larger Organizations or ISP's can get IP blocks from the American Registry for Internet Numbers
The following IP address ranges have been set aside for Internal use. These IP's are will not be routable on the Internet.
Class A: 10.xx.xx.xx
Class B:
172.16.xx.xx - 172.32.xx.xx
Class C: 192.168.xx.xx
Relevant RFC's
RFC 917 - Internet Subnets
RFC 950 - Internet Standard Subnetting Procedure
RFC 1517-Applicability Statement for the Implementation of Classless Inter-Domain Routing (CIDR)
RFC 1518- An Architecture for IP Address Allocation with CIDR
RFC 1519- Classless Inter-Domain Routing (CIDR): An Address Assignment and Aggregation Strategy
RFC 1520- Exchanging Routing Information Across Provider Boundaries in the CIDR Environment
RFC 1878 - Variable Length Subnet Table For IPv4