Chapter 11 - Perspectives on IPv4 Subnetting Flashcards
What rules apply to deciding which hosts should receive addresses from which subnet
- Addresses in the same subnet do not have to be separated by a router
- Addresses in different subnets that want to communicate must be separated by at least one router
True or False. In order for two devices to communicate over a link they must have addresses in the same subnet applied to the interfaces that connect to that link.
True. If you are using a technology such as PPPoE however this is not the case.
Finish. You should have a subnet for every ___, ___, and ___ connected to a network.
- VLAN
- P2P Serial Link
- Ethernet WAN
Define Subnet Mask
A subnet mask defines the size of a subnet. The formula for working this out is 2^H - 2 where H = Host Bits. The lowest value address in a subnet is known as the subnet number (network address) and the highest value address is known as the broadcast address.
How would you decide on the size of a subnet to use for a network?
You need to decide how many bits of an IP address need to be Host bits. For example if you had a network that requires 250 host addresses you would use the 2^H-2 equation until you found the correct number of Host bits (e.g. 2^6 - 2 = 62, 2^7 = 126, 2^8 - 2 = 254). In this case the smallest number of host bits you could use is 8.
What is the formula for working out the necessary size of a subnet?
2^H - 2 (H being host bits)
What does VLSM mean?
- Variable-Length Subnet Masks
- Allows a network to use different subnet mask lengths instead of a network design instead of having to have all of their subnets the same size
What are the main methods that the world is using to attempt to conserve IPv4 address space?
- IPv6 which uses 128 bit addresses instead of 32 bit like IPv4
- Assigning a subset of a public IP network to each company instead of an entire public IP network using CIDR (Classless Inter-Domain Routing). CIDR also allows for route aggregation to stop routing tables becoming too large.
- NAT (Network Address Translation) which allows the use of private IP networks inside organisations behind public addresses. Upto 65535 separate sessions can be supported by a single address.
How many networks can be made from each private address range?
Class A - 10.0.0.0 - 1 network
Class B - 172.16.0.0 -172.31.0.0 - 16 networks
Class C - 192.168.0.0 - 192.168.255.0 - 256 networks
What are the benefits of private networks in relation to improving the implementation of IPv4?
- Avoid using another organisation’s public address range for private networks - Certain parts of some networks require no internet access but still require IP addresses. Using public addresses for these parts would be pointless so it would be best to use private addresses and save public address space.
- Delay IPv4 address exhaustion - Organisations will not need to use their public addresses for all devices as NAT allows single public addresses to be translated into entire private networks.
- Reduce internet routers’ routing table size - Routers in the internet do not need routes for private IP networks used in organisations, only the public addresses.
What is RFC1918?
RFC 1918 (Address Allocation for Private Networks) is the request for comment that details methods of assigning private addresses to networks (e.g. NAT). It also details the reserved private IP address ranges that can be assigned to private networks.
- 10.0.0.0 - 10.255.255.255
- 172.16.0.0 - 172.31.255.255
- 192.168.0.0 - 192.168.255.255
How many host and network bits are there for each unsubnetted address class?
When unsubnetted:
- Class A has 8 Network bits (1 octet) and 24 Host bits (3 octets) - 16,777,214 Host addresses
- Class B has 16 Network bits (2 octets) and 16 Host bits (2 octets) - 65,534 Host addresses
- Class C has 24 Network bits (3 octets) and 8 Host bits (1 octet) - 254 Host addresses
True or False. In order to subnet a classful network, bits are borrowed from the Network part of the address and given to the Subnet part.
False. Bits cannot be borrowed from the Network part or added to the length of the entire 32 bit address. They are instead borrowed from the Host part.
What is the formula for working out how many subnets you will need?
2^S (S being subnet bits)
What is a subnet mask (Written as binary)?
- 32 bit binary string that determines the Network, Subnet, and Host addresses in a subnet.
- Defines Network bits as 1s, Subnet bits as 1s, and Host bits as 0s.
- For example if you had 16 Network bits, 8 Subnet bits, and 8 Host bits, you would write this as 16 1s (for Network) 8 1s (for Subnet) and 8 0s (for Host).
