155 lines
5 KiB
Markdown
155 lines
5 KiB
Markdown
---
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tags: [networks, internet]
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created: Friday, August 16, 2024
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---
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# IP addresses
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## IPv4 vs IPv6
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IP addresses come in two formats and bit-lengths depending on which version of
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the [Internet Protocol](Internet_Layer_of_Internet_Protocol.md) is being used.
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- IPv4 addresses are 32-bits in length
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- IPv6 addresses are 128-bits in length
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The larger the bit-length, the more unique addresses that can be generated. Thus
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IPv6 can create more addresses that IPv4.
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This entry focuses on IPv4 addresses.
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## Anatomy of an IPv4 address
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Each of the 32 bits are divided into four groups which are each 8 bits in
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length. Each group therefore comprises an octet and is separated by dots.
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The decimal number system, rather than binary is used to represent the values.
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This means each binary octet is three decimal digits in length.
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The 32 bits comprise two major groupings
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- the **network prefix**
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- the **host**
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Devices connected to the same network share the same network prefix. They are
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said to be on the same **subnet**. However each device will have a unique value
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for the host octet.
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(This account is somewhat idealised. The length of the prefix does not always
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align with the octet boundary. For instance, the prefix could be 25 bits in
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length, rather than 24. In this case it would "steal" one bit from the host
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section making it 23 bits in length.)
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Any changes to the length of the network prefix change the number of hosts
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available on the subnet. If the network prefix is longer, the number of unique
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hosts is reduced. If the network prefix is shorted, the number of unique hosts
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is increased.
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## Identifying the network and host groupings
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Given that the network prefix and host identifier do not always stick to set
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boundaries, we require a way of distinguishing the two values. There are two
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dominant methods.
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### CIDR notation
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This stands for _Classless Inter-Domain Routing_. An example:
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> 192.168.1.23/24
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The value after the forward slash is the number of bits that specify the network
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address. In the example, it is the standard 24-bits. Thus 192.168.1.23 stands
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for the network.
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### Subnet masking
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The subnet mask is another number _in addition to_ the network address. It is a
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kind of superimposed map on top of the address.
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Here is an example of a subnet mask:
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> 11111111.11111111.11111111.00000000
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The denary form would be as follows (as 255 is the maximal decimal number that
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can be represented with a single 8-bit number and 0 is the smallest):
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> 255.255.255.0
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In the binary form, the 1 values represent the bits which designate the network
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address and the 0 values represent the bits that designate the host.
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In the example above this corresponds to the idealised 32-bit:8-bit ratio of
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CIDR /24.
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There is a clever consequence of the subnet mask: if you apply a bitwise AND
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operator against the IP address and mask (both in their binary form) you can
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determine whether two addresses are on the same network.
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To compare the IP address 192.168.1.23 against 192.168.1.100 to demonstrate:
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```
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192.168.1.23:
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IP: 11000000.10101000.00000001.00010111
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Mask: 11111111.11111111.11111111.00000000
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Result: 11000000.10101000.00000001.00000000
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___________________________________
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192.168.1.100:
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IP: 11000000.10101000.00000001.00010111
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Mask: 11111111.11111111.11111111.00000000
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Result: 11000000.10101000.00000001.00000000
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```
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After applying the bitwise AND logic we see that the result is identical for
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both IPs indicating they are on the same network (share the same network
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prefix), whilst the host value is "masked".
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Here is a scenario where the subnet mask indicates that two IPs are not on the
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shared network:
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```
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192.168.1.23:
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IP: 11000000.10101000.00000001.00010111
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Mask: 11111111.11111111.11111111.00000000
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Result: 11000000.10101000.00000001.00000000
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___________________________________
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192.168.2.1
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IP: 11000000.10101000.00000010.00000001
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Mask: 11111111.11111111.11111111.00000000
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Result: 11000000.10101000.00000010.00000000
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```
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255.255.255.0 is not the only possible subnet mask. There are masks
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corresponding to ratios other than CIDR /24. For example:
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- 255.0.0.0 (/8)
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- 255.255.0.0 (/8)
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- 255.255.255.192 (/26)
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- 255.255.255.240 (/28)
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As noted previously, the different ratios will obviously affect the number of
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unique hosts available on the subnet.
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## Determining the range of hosts: practical example
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Let's say we have the following IP address expressed in CIDR: 192.168.0.133/27.
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In binary this would be:
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```
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110000000.10101000.00000000.100--00101
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```
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The `--` indicates the demarcation point between the network prefix and the bits
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designated for the host. 32 - 27 leaves us 5 bits for our range of hosts. This
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gives us 2^5 = 32 unique host values.
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In fact it will actually be 30 values since the first value (`00000`) will be
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used to identify the network itself and the last value (`11111`) will be the
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broadcast address - the address used when a message needs to be sent to all
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hosts on the network.
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An actual host address in this range is included in the example: `00101`.
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