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Level 13

Analyzing Subnet Masks


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prefix
Although the DDN format has been around since the beginning of IPv4 addressing, the third mask format was added later, in the early 1990s: the __ format.
slash /
Prefix format lists a __ followed by the number of binary 1s in the binary mask.
prefix mask, CIDR mask, slash mask
Note that although the terms prefix or __ can be used, the terms __ or __ can also be used.
Binary to prefix
Count the number of binary 1s in the binary mask, and write the total, in decimal, after a /.
Prefix to binary
Write P binary 1s, where P is the prefix value, followed by as many binary 0s as required to create a 32-bit number.
0
Binary Mask Octet: [00000000]
128
Binary Mask Octet: [10000000]
192
Binary Mask Octet: [11000000]
224
Binary Mask Octet: [11100000]
240
Binary Mask Octet: [11110000]
248
Binary Mask Octet: [11111000]
252
Binary Mask Octet: [11111100]
254
Binary Mask Octet: [11111110]
255
Binary Mask Octet: [11111111]
Binary to decimal
Organize the bits into four sets of eight. For each octet, find the binary value in the table and write down the corresponding decimal value.
Decimal to binary
For each octet, find the decimal value in the table and write down the corresponding 8-bit binary value.
binary
The best way to convert between the prefix and decimal formats is to first convert to __.
24 bits
In this case, the subnets use a subnet mask of /24, which means that the prefix part of the addresses is __ (3 octets) long.
subnet
The prefix part must be the same value for all addresses in a __.
prefix
The subnet mask subdivides the IP addresses in a subnet into two parts: the __, or subnet part, and the host part.
group, member
The prefix or subnet part of the address identifies the __, and the host part identifies the specific __ of the group.
dividing line
The subnet mask defines the __ between the prefix and the host part.
network part, subnet part
The class defines the length of the __, with the __ simply being the rest of the prefix.
network, subnet
The combined __ and __ parts act like the prefix because all addresses in the same subnet must have identical values in the __ and __ parts.
24 – 8 = 16
The class defines 8 network bits, and the mask defines 24 prefix bits, meaning that __ subnet bits exist.
Classful addressing
The concept that an IPv4 address has three parts—network, subnet, and host—as defined by the mask and Class A, B, and C rules.
Classless addressing
The concept that an IPv4 address has two parts—the prefix part plus the host part—as defined by the mask, with no consideration of the class (A, B, or C).
P + H = 32, N + S + H = 32
Keeping in mind that IPv4 addresses are 32 bits long, the two parts with classless addressing must add up to 32 (__), and with classful addressing, the three parts must add up to 32 (__).
16
[1] 255.255.0.0, so P=__.
A, 8
[2] 8.1.4.5 is in the range 1–126 in the first octet, so it is Class __; so N=__.
Step 1
Convert the mask to prefix format (/P) as needed.
Step 2
Determine N based on the class.
Step 3
Calculate S = P – N.
Step 4
Calculate H = 32 – P.
Step 5
Calculate hosts/subnet: 2H – 2.
Step 6
Calculate number of subnet: 2S.
Level 14