What subnet mask is represented by the slash notation /20?
255.255.255.248
255.255.224.0
255.255.240.0
255.255.255.0
255.255.255.192
The correct answer and explanation is :
The subnet mask represented by the slash notation /20 is 255.255.240.0. This is determined by converting the CIDR (Classless Inter-Domain Routing) notation to its corresponding dotted-decimal subnet mask.
Understanding CIDR Notation:
CIDR notation specifies the number of bits used for the network portion of an IP address. In the case of /20, it indicates that the first 20 bits are allocated for the network address, leaving the remaining 12 bits for host addresses within that network.
Converting /20 to Dotted-Decimal Notation:
- Binary Representation:
- A standard IPv4 address is 32 bits long, divided into four octets (8 bits each).
- A /20 subnet mask means the first 20 bits are set to 1, and the remaining 12 bits are set to 0.
- This results in the following binary sequence:
11111111.11111111.11110000.00000000
- Decimal Conversion:
- Convert each octet from binary to decimal:
- First octet: 11111111 → 255
- Second octet: 11111111 → 255
- Third octet: 11110000 → 240
- Fourth octet: 00000000 → 0
- Thus, the subnet mask in decimal form is 255.255.240.0.
Matching with Provided Options:
Among the given options:
- 255.255.255.248
- 255.255.224.0
- 255.255.240.0
- 255.255.255.0
- 255.255.255.192
The correct match is 255.255.240.0, corresponding to the /20 subnet mask.
Explanation of Other Options:
- 255.255.255.248 (/29):
- Provides 6 host addresses (2^3 – 2).
- 255.255.224.0 (/19):
- Provides 8,190 host addresses (2^13 – 2).
- 255.255.255.0 (/24):
- Provides 254 host addresses (2^8 – 2).
- 255.255.255.192 (/26):
- Provides 62 host addresses (2^6 – 2).
These options correspond to different subnet sizes and do not match the /20 subnet mask.
Summary:
A /20 subnet mask translates to 255.255.240.0 in dotted-decimal notation. This subnet mask allows for 4,096 IP addresses (2^12), with 4,094 usable for hosts after reserving the network and broadcast addresses. This configuration is commonly used in networks requiring a moderate number of host addresses.