What Is A Subnet

What is a Subnet? A subnet is a logical organization of network address ranges used to separate hosts and network devices from each other to serve a design purpose. In many cases, subnets are created to serve as physical or geographical separations similar to those found between rooms, floors, buildings, or cities. Most modern subnet definitions are created according to 3 main factors. These include: the number of hosts that needs to exist on the subnet now and in the future; the necessary security controls between networks; and the performance required for communications between hosts. Legacy Subnets Legacy subnets were not flexible because they had predefined limitations on their size and numbers. These were called "classful" networks because each network could be easily identified and placed into a specific class. Shown below is a table containing the original "classful" definitions for IP addresses: IP Address Range CIDR Equivalent Purpose RFC Class Total # of Addresses 0.0.0.0 - 0.255.255.255 0.0.0.0/8 Zero Addresses 1700 A 16,777,216 10.0.0.0 - 10.255.255.255 10.0.0.0/8 Private IP addresses 1918 A 16,777,216 127.0.0.0 - 127.255.255.255 127.0.0.0/8 Localhost Loopback Address 1700 A 16,777,216 169.254.0.0 - 169.254.255.255 169.254.0.0/16 Zeroconf / APIPA 3330 B 65,536 172.16.0.0 - 172.31.255.255 172.16.0.0/12 Private IP addresses 1918 B 1,048,576 192.0.2.0 - 192.0.2.255 192.0.2.0/24 Documentation and Examples 3330 C 256 192.88.99.0 - 192.88.99.255 192.88.99.0/24 IPv6 to IPv4 relay Anycast 3068 C 256 192.168.0.0 - 192.168.255.255 192.168.0.0/16 Private IP addresses 1918 C 65,536 198.18.0.0 - 198.19.255.255 198.18.0.0/15 Network Device Benchmark 2544 C 131,072 224.0.0.0 - 239.255.255.255 224.0.0.0/4 Multicast 3171 D 268,435,456 240.0.0.0 - 255.255.255.255 240.0.0.0/4 Reserved 1700 E 268,435,456 Classless IP Addresses With the advent of CIDR (Classless Inter-Domain Routing), the "classful" definition of subnet divisions was lifted. Any network address could be defined just as any of the "classful" subnet of the past could be defined. All that is required is enough neighboring

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address space to cover all the IP addresses needed. Classless addresses also assist in reducing the overall size of the global routing tables on network devices.

What is Subnetting? Subnetting is the process of breaking down an IP network into smaller sub-networks called "subnets." Each subnet is a non-physical description (or ID) for a physical subnetwork (usually a switched network of host containing a single router in a multi-router network).

Subnet Mask Notation There are two forms of subnet notation, standard notation and CIDR (Classless Internet Domain Routing) notation. Both versions of notation use a base address (or network address) to define the starting point of the network, such as 192.168.1.0. This means that the network begins at 192.168.1.0 and the first possible host IP address on this subnet would be 192.168.1.1. In standard subnet mask notation, a four octet numeric value is used as with the base address, for example 255.255.255.0. The standard mask can be calculated by creating four binary values for each octet, and assigning the binary digit of .1. to the network potion, and assigning the binary digit of .0. to the network potion. In the example above this value would be 11111111.11111111.11111111.00000000. In combination with the base address you have a subnet definition, in this case the subnet in standard notation would be 192.168.1.0 255.255.255.0. In CIDR notation, the number of 1.s in the binary version of the mask are counted from the left, and that number is appended to the end of the base address following a slash (/). In the example here the subnet would be listed in CIDR notation as 192.168.1.0/24.

When is Subnetting Used? Subnets are created to limit the scope of broadcast traffic, to apply network security measures, to separate network segments by function, and/or to assist in resolving network congestion problems.., A subnet is usually composed of a network router, a switch or hub, and at least one host

How can I Compute the Maximum Number of Hosts for a Subnet Mask? To compute the maximum number of hosts for a subnet mask, take two and raise it to the amount of bits allocated to the subnet (count the number of 0.s in the subnet mask binary value) and subtract two. You must subtract two from the resulting value because the first value in the IP address range (all 0s) is reserved for the network address, and the last value in the IP address range (all 1s) is reserved for the network broadcast address. For example, DSL networks commonly use 8 bits for their subnets. The amount of allowable

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hosts for such a DSL network could be computed by the following formula: max hosts = (2^8)-2 = 254 hosts. As you subnet networks, the number of bits represented by the subnet mask will decrease. You decrease the octets in order starting from the rightmost value and proceed left as you reach a zero value. Mask values decrease by a power of two each time you split a network into more subnets. Values are 255, 254*, 252, 248, 240, 224, 224, 192, 128. Each decrease indicates that an additional bit has been allocated. After 128, the next bit allocated will reduce the fourth octet to 0; and the third octet will follow same 8-number progression. For instance, a subnet mask dotted decimal number of 255.255.255.255 indicates that no bits have been allocated and that the maximum number of hosts is 1 (0^1=1). The subnet mask 255.255.255.128 indicates that the maximum number of hosts is 128. And the subnet mask 255.255.128.0 indicates that the maximum number of hosts is 32,786. 254 is not a valid number for the fourth octet because no addresses are available for hosts. i.e. (2^1)-2 = 0.

What is a Subnet Mask? A subnet mask allows you to identify which part of an IP address is reserved for the network, and which part is available for host use. If you look at the IP address alone, especially now with classless inter-domain routing, you can't tell which part of the address is which. Adding the subnet mask, or netmask, gives you all the information you need to calculate network and host portions of the address with ease. In summary, knowing the subnet mask can allow you to easily calculate whether IP addresses are on the same subnet, or not.

Determining network and host portions of an IP address using a subnet mask To determine what the network address is for any given IP address, you merely have to convert both octal addresses into binary, and do a bitwise AND operation. An example using an IP address of 156.154.81.56 used with a network mask of 255.255.255.240 follows:IP Address: 10011100.10011010.01010001.00111000 Subnet mask: 11111111.11111111.11111111.11110000 Bitwise AND ----------------------------------------------Result: 10011100.10011010.01010001.00110000 As you can see, the network address for the IP address and subnet mask in question is 156.154.81.48. To determine the how many hosts are possible to be on this same subnet, it is a simple operation. Count the number of bits from the right until you get to the first "1" in the binary network address display. That number will be the power you raise 2 to for the calculation of possible number of hosts. You must also subtract

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two from the result because one address is reserved for broadcast and network addresses. This leaves you with the final algorithm of 2^n-2. In this case there are 4 bits of 0 in the network address, leaving you with 2^4-2 hosts possible, or 14 hosts. This means that your network address is 156.54.81.48, that you have a range of addresses available to hosts from 156.154.81.49 - 156.154.81.62, and that the broadcast address for this network is 156.154.81.63.

Are subnet masks necessary? Subnet masks are critical to communications on an IP network. Network devices use the IP address targets and defined netmask to determine if the network the host is on is a local subnet, or a remote network. This is important because devices act differently depending on the result. If the subnet is local, the device will send an ARP request to retrieve the MAC or hardware address of the system in question to communicate over the data-link layer. If the address is found to be on a remote network, then the network device routes packets to the gateway in it's routing table that is set to handle that network. If no routing table entry is found matching that network, the packets are routed to the default route. If no default route is defined, the packets are dropped with nowhere left to go.

What is a Broadcast Domain? Definitions of a Broadcast Domain A broadcast domain is a logical part of a network (a network segment) in which any network equipment can transmit data directly to another equipment or device without going through a routing device (assuming the devices share the same subnet and use the same gateway; also, they must be in the same VLAN). A more specific definition of the broadcast domain is that certain area of the computer network that's composed by every single computer or network equipment that can be reached directly by sending a simple frame to the data link layer's broadcast address.

Details on Broadcast Domains While any layer 2 device is able to divide the collision domains, broadcast domains are only divided by layer 3 network devices such as routers or layer 3 switches. Frames are normally addressed to a specific destination device on the network. While all devices detect the frame transmission on the network, only the device to which the frame is addressed actually receives it. A special broadcast address consisting of all 1s is used to send frames to all devices on the network. The VLAN (Virtual Local Area Network) technology can also create a so-called "virtual" broadcast domain. A network built with switching devices could see each network devices as an independent system. These groups of independent systems can be joined into one broadcast domain, even if the computers are not physically

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connected to each other. This is very useful when administrating large networks where there's the need for better network management and control.

How to restrict the broadcast domain? Since a broadcast domain is the area where broadcasts can be received, broadcasts are restricted by routers. If a router receives a broadcast signal, it simply drops it. In other words, the edge or border router connected to the Internet will not up-broadcast or will not relay that broadcast message. This is problematic and not foolproof either. Supposing two networks exist and they're connected to each other through a router. The first network has a running DHCP server that offers IP addresses to networked systems. On the other side, there is no valid DHCP server running on the second network. Offering IP addresses from the first network's DHCP server to the second network's systems can be a difficult task to accomplish since DHCP is a broadcast, and the router that joins the networks drops the broadcast traffic. This leaves any DHCP request in the second network, unanswered. Many router manufacturers provide capabilities for DHCP forwarding to solve this problem. This can be bypassed by connecting the two networks with a well configured, Linux-based, purpose-oriented software router. That will handle the job properly and without further issues.

Examples of Broadcast Domain Implementation A broadcast domain can be perceived as a portion of a network that is reachable by network broadcast (an address resolution request of some sort of an announcement of availability. Messages like "Can I have the address of that network node?" or "I'm here, a part of this network". Again, unlike collision domains, broadcast domains can be started at several protocol layers, therefore the broadcast domain as a whole can be within a single LAN segment but it can be routed over other segments as well.

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