Which octet is subnet

Borrow 3 bits - So 2 raised to 3 rd 2 3 is 8 possible subnet address combinations. The binary would look like this : My 8 subnet addresses would be :. The remaining 5 digits in each subnet are possible host positions. So 2 to the 5 th is 32 potential hosts. In the first potential subnet they would look like this. Any IP address whose first octet is between 1 and is a Class A address.

Note that 0 is reserved as a part of the default address, and is reserved for internal loopback testing. Any address whose first octet is in the range to is a Class B address. The first octet range of to is a Class C address. Multicast IP addresses have their first octets in the range to It was needed because of the rapid growth of the Internet and growth of the IP routing tables held in the Internet routers. In this case, DeviceA belongs to subnet Given the Class C network of Looking at the network shown in Figure 3 , you can see that you are required to create five subnets.

The largest subnet must support 28 host addresses. Is this possible with a Class C network? And if so, then how? You can start by looking at the subnet requirement. In order to create the five needed subnets you would need to use three bits from the Class C host bits. Two bits would only allow you four subnets 2 2. Since you need three subnet bits, that leaves you with five bits for the host portion of the address.

How many hosts does this support? This meets the requirement. Therefore you have determined that it is possible to create this network with a Class C network. An example of how you might assign the subnetworks is:. In all of the previous examples of subnetting, notice that the same subnet mask was applied for all the subnets. This means that each subnet has the same number of available host addresses. You can need this in some cases, but, in most cases, having the same subnet mask for all subnets ends up wasting address space.

For example, in the Sample Exercise 2 section, a class C network was split into eight equal-size subnets; however, each subnet did not utilize all available host addresses, which results in wasted address space. Figure 4 illustrates this wasted address space. Figure 4 illustrates that of the subnets that are being used, NetA, NetC, and NetD have a lot of unused host address space. It is possible that this was a deliberate design accounting for future growth, but in many cases this is just wasted address space due to the fact that the same subnet mask is used for all the subnets.

Given the same network and requirements as in Sample Exercise 2 develop a subnetting scheme with the use of VLSM, given:. The easiest way to assign the subnets is to assign the largest first. For example, you can assign in this manner:. Figure 5 illustrates how using VLSM helped save more than half of the address space.

Classless Interdomain Routing CIDR was introduced in order to improve both address space utilization and routing scalability in the Internet. It was needed because of the rapid growth of the Internet and growth of the IP routing tables held in the Internet routers.

Length means the number of left-most contiguous mask bits that are set to one. So network CIDR also depicts a more hierarchical Internet architecture, where each domain takes its IP addresses from a higher level.

This allows for the summarization of the domains to be done at the higher level. For example, if an ISP owns network Yet, when advertising to other providers, the ISP only needs to advertise A bit subnet mask allows for four IPv4 addresses: two host addresses, one all-zeros network, and one all-ones broadcast address.

A point-to-point link can only have two host addresses. There is no real need to have the broadcast and all-zeros addresses with point-to-point links. A bit subnet mask will allow for exactly two host addresses, and eliminates the broadcast and all-zeros addresses, thus conserving the use of IP addresses to the minimum for point-to-point links.

However, they can also be used on broadcast interface types like ethernet interfaces. If that is the case, make sure there are only two IPv4 addresses needed on that ethernet segment. Some administrators will only use two of the subnetworks using For more information on this topic, see RFC In these four networks, the last six binary digits can be used for host addresses. Using a subnet mask of These four networks would have as valid host addresses:. Remember, again, that binary host addresses with all ones or all zeros are invalid, so you can't use addresses with the last octet of 0, 63, 64, , , , , or You can see how it works by looking at two host addresses, If you used the default Class C subnet mask of However, if you use the subnet mask of The result of this comparison tells the computer whether the destination is a local host or a remote host.

If the result of this process determines the destination to be a local host, then the computer will send the packet on the local subnet. It's then the responsibility of the router to forward the packet to the correct subnet. Incorrect Subnet Mask: If a network uses a subnet mask other than the default mask for its address class, and a client is still configured with the default subnet mask for the address class, communication will fail to some nearby networks but not to distant ones.

As an example, if you create four subnets such as in the subnetting example but use the incorrect subnet mask of In this situation, packets destined for hosts on different physical networks that are part of the same Class C address won't be sent to a default gateway for delivery. A common symptom of this issue is when a computer can communicate with hosts that are on its local network and can talk to all remote networks except those networks that are nearby and have the same class A, B, or C address.

Incorrect IP Address: If you put computers with IP addresses that should be on separate subnets on a local network with each other, they won't be able to communicate. They'll try to send packets to each other through a router that can't forward them correctly. A symptom of this problem is a computer that can talk to hosts on remote networks, but can't communicate with some or all computers on their local network.

To correct this problem, make sure all computers on the same physical network have IP addresses on the same IP subnet. If you run out of IP addresses on a single network segment, there are solutions that go beyond the scope of this article.

Incorrect Default Gateway: A computer configured with an incorrect default gateway can communicate with hosts on its own network segment. But it will fail to communicate with hosts on some or all remote networks. A host can communicate with some remote networks but not others if the following conditions are true:. Internet--The global collection of networks that are connected together and share a common range of IP addresses.

Network--There are two uses of the term network in this article. One is a group of computers on a single physical network segment. The other is an IP network address range that is allocated by a system administrator. Octet--An 8-bit number, 4 of which comprise a bit IP address.

They have a range of that correspond to the decimal values