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Perspectives on IPv4 Subnetting

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IP subnet
An __ is simply a subset of a Class A, B, or C network. If fact, the word subnet is a shortened version of the phrase subdivided network.
Most IT jobs require you work with __ from an operational view. That is, someone else, before you got the job, designed how IP addressing and subnetting would work for that particular enterprise network.
Analyze Needs
# Subnets, # Hosts/Subnet, 1 Size Subnet
Design Subnets
Choose Network, Choose 1 Mask, List All Subnets
Plan Implementation
Subnets -> Locations, Static IP, DHCP Ranges
Analyze Subnetting and Addressing Needs
Which hosts should be grouped together into a subnet?
Analyze Subnetting and Addressing Needs
How many subnets does this network require?
Analyze Subnetting and Addressing Needs
How many host IP addresses does each subnet require?
Analyze Subnetting and Addressing Needs
Will we use a single subnet size for simplicity, or not?
Addresses in the same subnet are not separated by a __.
different subnets
Addresses in __ are separated by at least one router.
same link
Hosts on the __ must be in the same subnet is much like the postal code concept. All mailing addresses in the same town use the same postal code.
one per interface, different subnets
Router R1 connects to one LAN subnet on the left and one WAN subnet on the right. To do so, R1 will be configured with two different IP addresses, __. These addresses will be in __, because the interfaces connect the router to __.
MPLS, Frame Relay
WAN technologies like __ and __ allow subnetting options other than one subnet per pair of routers on the WAN.
reasonable estimate
Any subnetting plan should include a __ of the number of subnets you need to meet future needs.
research, questioning
Determining the number of hosts per subnet requires knowing a few simple concepts and then doing a lot of __ and __.
business plans
Every device that connects to a subnet needs an IP address. For a totally new network, you can look at __, numbers of people at the site, devices on order, and so on.
bigger, smaller
When expanding an existing network to add new sites, you can use existing sites as a point of comparison, and then find out which sites will get __ or __.
Don’t forget to count the router __ IP address in each subnet and the switch IP address used to remotely manage the switch.
The final choice in the initial planning step is to decide whether you will use a simpler design by using a __ philosophy.
usable IP addresses
A subnet’s size, or length, is simply the number of __ in the subnet.
one size subnet, varied sizes
A subnetting design can either use __, or __ of subnets, with pros and cons for each choice.
subnet mask
The engineer assigns each subnet a __, and that mask, among other things, defines the size of that subnet.
host bits
The mask sets aside a number of __ whose purpose is to number different host IP addresses in that subnet.
2^H – 2
However, the subnet’s size is not 2H. It’s __, because two numbers in each subnet are reserved for other purposes.
subnet number, subnet broadcast address
Each subnet reserves the numerically lowest value for the __ and the numerically highest value as the __.
subnet number, subnet ID, subnet address
The terms __, __, and __ all refer to the number that represents or identifies a subnet.
single-size subnet
To choose to use a __ in an enterprise network, you must use the same mask for all subnets, because the mask defines the size of the subnet.
Operational simplicity
What’s the big advantage when using a single-size subnet?
The big disadvantage for using a single-size subnet is that it __ IP addresses.
variable-length subnet masks
You still need to be ready to work with __, which is the practice of using different masks for different subnets in the same classful IP network.
subnetting design
now that you know how many subnets you need and how many host addresses you need in the largest subnet, how do you create a useful __ that meets those requirements?
registered public IP network
The original design of the Internet required that any company that connected to the Internet had to use a __.
public IP network
In particular, after a __ is assigned to a company, only that company should use the addresses in that network.
no duplicate
That guarantee of uniqueness means that Internet routing can work well, because there are __ public IP addresses.
original address assignment process
This __ ensured unique IP addresses across the entire planet.
early 1990s
By the __, the world was running out of public IP networks that could be assigned.
IANA, early 2011
__, which assigns public IPv4 address blocks to the five Internet registries around the globe, assigned the last of the IPv4 address space in __.
IPv6, 128 bit
A new version of IP (__), with much larger addresses (__)
subset of a public IP network, waste
Assigning a __ to each company, instead of an entire public IP network, to reduce __.
__, which allows the use of private IP networks.
__, allows multiple companies to use the exact same private IP network, using the same IP addresses as other companies, while still connecting to the Internet.
private IP networks
For now, accept that most companies use NAT, and therefore, they can use __ for their internetworks.
RFC 1918
__ defines the set of private IP networks.
Private IP Networks
Will never be assigned to an organization as a public IP network
Private IP Networks
Can be used by organizations that will use NAT when sending packets into the Internet
Private IP Networks
Can also be used by organizations that never need to send packets into the Internet
The company can simply pick one or more of the private IP networks from the list of __ private IP network numbers.
B through
C through
Number of Private IP Networks in class A
Number of Private IP Networks in class B
Number of Private IP Networks in class C
private IP addresses
Most new enterprise internetworks use __ throughout the network, along with NAT, as part of the connection to the Internet.
Before an engineer __ a classful network, the network is a single group of addresses.
network part, host part
When thinking about an unsubnetted classful network, the addresses in a network have only two parts: the __ and __.
Class A
Class B
Class C
subnet part
To subnet a network, the designer thinks about the network and host parts, and then the engineer adds a third part in the middle: the __.
host part
To create a subnet part of the address structure, the engineer borrows bits from the __.
network bits, 8 16 24
N, representing the number of __, remains locked at __, __, or __, depending on the class.
dividing line, subnet bits, host bits
The designer moves a (dashed) __ into the host field, with __ (S) between the network and host parts, and the remaining __ (H) on the right.
subnet part
The bits in the __ create a way to uniquely number the different subnets that the design engineer wants to create.
2 4 8
With 1 subnet bit, you can number __ subnets. With 2 bits, __ subnets, with 3 bits, __ subnets, and so on.
large enough
The number of subnet bits must be __ to uniquely number all the subnets, as determined during the planning process.
host bits
At the same time, the remaining number of __ must also be large enough to number the host IP addresses in the largest subnet.
single mask
The idea of calculating the number of subnets as 2^S applies only in cases where a __ is used for all subnets of a single classful network.
To effectively design masks, or to interpret masks that were chosen by someone else, you need a good working memory of the powers of __.
How many subnet (S) bits do I need to number 200 subnets?
How many host (H) bits do I need to number 200 hosts per subnet?
subnet mask
Although engineers think about IP addresses in three parts when making design choices (network, subnet, and host), the __ gives the engineer a way to communicate those design choices to all the devices in the subnet.
subnet mask, left, right
The __ is a 32-bit binary number with a number of binary 1s on the __ and with binary 0s on the __.
Because the network part always comes first, then the subnet part, and then the host part, the subnet mask, in binary form, cannot have __ 1s and 0s.
unbroken string
Each subnet mask has one __ of binary 1s on the left, with the rest of the bits as binary 0s.
N 1s, S 1s, H 0s
After the engineer chooses the classful network and the number of subnet and host bits in a subnet, creating the binary subnet mask is easy. Just write down __, __, and then __.
DDN, prefix
Binary masks can also be written in two other formats: the familiar __ seen in IP addresses and an even briefer __ notation.
group of consecutive
A subnet consists of a __ numbers. Most of these numbers can be used as IP addresses by hosts.
first, last
Each subnet reserves the __ and __ numbers in the group, and these two numbers cannot be used as IP addresses.
Subnet number
This number identifies the subnet. It is the numerically smallest number in the subnet. It cannot be used as an IP address by a host.
Subnet broadcast
This is the last (numerically highest) number in the subnet. It also cannot be used as an IP address by a host.
IP addresses
All the numbers between the subnet ID and the subnet broadcast address can be used as a host IP address.
For DHCP to work, the network engineer must tell the DHCP server the __ for which it must assign IP addresses.
Level 12