1.7d - IPv4 Subnet Masks (Prof. Messer)

Practice flashcards covering classless subnetting, CIDR block notation, and the relationship between binary and decimal subnet mask representations based on CompTIA Network+ N10-009.

Classless Subnetting

A method of IP addressing adopted since $1993$ that allows for subnet masks that do not follow fixed Class A, Class B, or Class C arrangements.

Classless Interdomain Routing (CIDR)

Standard used for classless subnetting that refers to subnet masks by the number of bits rather than decimal form.

CIDR block notation

A syntax represented by a slash followed by the number of network bits, such as $/24$, to specify a subnet mask.

Subnet Mask (Binary)

A contiguous series of $1$'s on the left, used to identify the network portion, followed by $0$'s on the right, used to identify the host portion.

Network section/ID

The part of an IP address defined by the bits set to $1$ in the subnet mask.

Host ID

The part of an IP address defined by the bits set to $0$ in the subnet mask, which are available for assignable IP addresses.

$/8$ CIDR notation

A subnet mask with eight individual bits in the first octet, equivalent to $255.0.0.0$ in decimal.

$/24$ CIDR notation

A subnet mask with $24$ network bits ($8+8+8$), equivalent to the decimal mask $255.255.255.0$.

$/16$ CIDR notation

A subnet mask where the first $16$ bits represent the network and the last $16$ bits represent the host, equivalent to $255.255.0.0$.

$/26$ CIDR notation

A subnet mask where the first $26$ bits are the network part and the last $6$ bits are reserved for the host ID, equivalent to $255.255.255.192$.

$/12$ CIDR notation

A subnet mask where the first $12$ bits are for the network ID and the last $20$ bits are for the host ID, equivalent to $255.240.0.0$.

$/19$ CIDR notation

A subnet mask with $19$ network bits and $13$ host bits, equivalent to the decimal representation $255.255.224.0$.

$/20$ CIDR notation

A subnet mask with $20$ network bits and $12$ host bits, equivalent to the decimal representation $255.255.240.0$.

Binary $10000000$

An octet with one bit set to $1$, equivalent to the decimal value $128$.

Binary $11000000$

An octet with two bits set to $1$, equivalent to the decimal value $192$.

Binary $11100000$

An octet with three bits set to $1$, equivalent to the decimal value $224$.

Binary $11110000$

An octet with four bits set to $1$, equivalent to the decimal value $240$.