1 / 16

Chapter 12

Chapter 12. Subnetting CIS 1140. Subnetting. Subnetting: taking a single network address issued from a registrar or an ISP and creating two or more subnetworks A subnet is a network created by borrowing bits from the host portion of an assigned network IP address.

isra
Télécharger la présentation

Chapter 12

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 12 Subnetting CIS 1140

  2. Subnetting • Subnetting: taking a single network address issued from a registrar or an ISP and creating two or more subnetworks • A subnet is a network created by borrowing bits from the host portion of an assigned network IP address. • Binary number system is used for IP addressing. • Even though binary number system is used for addressing, IP addresses are typically written in dotted decimal format. • Ex. 172.16.15.3

  3. Subnetting (cont.) • 1. Determine the class of the IP address to be used and all accompanying information: • 0 – 127 Class A • N – H – H – H • 255.0.0.0 • 128 – 191 Class B • N – N – H – H • 255.255.0.0 • 192 – 223 Class C • N – N – N – H • 255.255.255.0

  4. Subnetting (cont.) • 2. Determine how many bits to “borrow” for subnetting based on the number of networks or hosts that must be accommodated • You must leave at least 2 bits for hosts. • 2n - 2 ≥ number of subnets needed (where n is the number of bits borrowed for subnetting) • 2n - 2 ≥ number of hosts needed (where n is the number of bits left for addressing hosts)

  5. Subnetting (cont.) • How many bits would need to be borrowed to produce 7 subnets? 2n - 2 ≥ 7 2n ≥ 9 n = 4 (remember 24=16) • How many bits would need to be borrowed to produce 25 subnets? 2n - 2 ≥ 25 2n ≥ 27 n = 5 (remember 25=32)

  6. Subnetting (cont.) • How many bits would need to be borrowed to produce 220 subnets? 2n - 2 ≥ 220 2n ≥ 222 n = 8 (remember 28=256) • How many bits would need to be used to produce 12 hosts per subnet? 2n - 2 ≥ 12 2n ≥ 14 n = 4 (remember 24=16)

  7. Subnetting (cont.) • How many bits would need to be used to produce 35 hosts per subnet? 2n - 2 ≥ 35 2n ≥ 37 n = 6 (remember 26=64) • How many bits would need to be used to produce 500 hosts per subnet? 2n - 2 ≥ 500 2n ≥ 502 n = 9 (remember 29=512)

  8. Subnetting (cont.) • How many bits are borrowed for subnetting a class B address if 10 must be used for hosts? 8 bits from the 3rd octet + 8 bits from the 4th octet give 16 bits for hosts + subnets 16 – 10 = 6 • How many bits are borrowed for subnetting a class C address if 3 must be used for hosts? 8 bits from the 4th octet give 8 bits for hosts + subnets 8 – 3 = 5

  9. Subnetting (cont.) • 3. Determine the actual subnet mask of the newly created networks • Start with the default subnet mask and add in the bits that were “borrowed” or set during the subnetting process • If three bits were borrowed from a class C address: 128 64 32 16 8 4 2 1 1 1 1 0 0 0 0 0 = 224 (128+64+32) The mask would be 255.255.255.224

  10. Subnetting (cont.) • If four bits are borrowed from a class C address: 128 64 32 16 8 4 2 1 1 1 1 1 0 0 0 0 = 240 (128+64+32+16) The mask would be 255.255.255.240 • If four bits are borrowed from a class B address: 128 64 32 16 8 4 2 1 1 1 1 1 0 0 0 0 = 240 (128+64+32+16) The mask would be 255.255.240.0

  11. Subnetting (cont.) • 4. Determine what value you are ranging on. The last bit turned on in an octet will determine the range. • Borrow 3 bits for subnetting 192.168.3.0 128 64 32 16 8 4 2 1 1 1 1 0 0 0 0 0 This would range on 32. 192.168.3.0 192.168.3.128 192.168.3.32 192.168.3.160 192.168.3.64 192.168.3.192 192.168.3.96 192.168.3.224 *Note the equation 2n - 2 ≥ subnets When n = 3 it is predicted that 6 useable subnets will be produced. When the first and last subnet are thrown out this is true. This would leave 5 bits for hosts. 2n - 2 ≥ hosts would predict 30 useable hosts which is also true.

  12. Subnetting (cont.) • The first address in a network or subnet is used to identify the subnet/network and cannot be used to address hosts. • The last address in a network or subnet is used as the broadcast for the subnet/network and cannot be used to address hosts. • When a subnet is created, we typically throw out the first and last subnet to avoid confusion: • In the previous example: • Does 192.168.3.0 identify the entire 192.168.3.0 network or just the first subnet? • Does 192.168.3.255 create a broadcast for the entire 192.168.3.0 network or just the subnet 192.168.3.224? • This throwing out of two subnets and two addresses in each subnet explains the – 2 in the equation 2n - 2 ≥ number of hosts/subnets

  13. Subnetting (cont.) • Borrow 4 bits for subnetting 192.168.3.0 128 64 32 16 8 4 2 1 1 1 1 1 0 0 0 0 This would range on 16. 192.168.3.0 192.168.3.128 192.168.3.16 192.168.3.144 192.168.3.32 192.168.3.160 192.168.3.48 192.168.3.176 192.168.3.64 192.168.3.192 192.168.3.80 192.168.3.208 192.168.3.96 192.168.3.224 192.168.3.112 192.168.3.240 *Note the equation 2n - 2 ≥ subnets When n = 4 it is predicted that 14 useable subnets will be produced. When the first and last subnet are thrown out this is true. This would leave 4 bits for hosts. 2n - 2 ≥ hosts would predict 14 useable hosts which is also true.

  14. Variable Length Subnet Mask • Variable Length Subnet Mask (VLSM): a subnet mask that is not expressed in standard 8-bit or one-byte values. A VLSM occurs when a subnet is further divided into smaller subnets, which are not equal to the original subnet in length or in the number of hosts. • Subnetting a subnet • Avoids wasting addresses when subnets vary greatly in size • Ex. Point-to-point links really require only 4 address: 1 ID 1 Broadcast 2 one address for each side of the link • Fixed Length subnet Mask (FLSM): subnets are equal in range and have an equal number of hosts

  15. Advantages of Subnetting • Creates a more secure network by placing hosts on separate networks. • Breaks up broadcast domains. Broadcast traffic from one network does not “bleed” over into other networks. • Fewer collision occur because of reduced broadcasts and segmentation reduces the size of the collision domains.

  16. Disadvantages of Subnetting • Subnetting can become confusing as networks grow larger. • Subnetting itself is not an easy process if the administrator does not practice. • Not all device and protocols can use all types of subnetting. • Ex. Not all routing protocols support VLSM.

More Related