What is a subnet mask and how is it used?
Theme: IP Addressing Role: Network Engineer Function: Technology
Interview Question for Network Engineer: See sample answers, motivations & red flags for this common interview question. About Network Engineer: Designs and maintains computer networks. This role falls within the Technology function of a firm. See other interview questions & further information for this role here
Sample Answer
Example response for question delving into IP Addressing with the key points that need to be covered in an effective response. Customize this to your own experience with concrete examples and evidence
- Definition of Subnet Mask: A subnet mask is a 32-bit number that is used to divide an IP address into network and host portions
- Purpose of Subnet Mask: The subnet mask is used to determine the network address of an IP address and to identify which part of the IP address represents the network and which part represents the host
- Binary Representation: A subnet mask is represented in binary form as a series of 1s followed by a series of 0s. The 1s represent the network portion, and the 0s represent the host portion
- Applying Subnet Mask: To apply a subnet mask to an IP address, each bit of the IP address is compared to the corresponding bit in the subnet mask. If the corresponding bit in the subnet mask is 1, it is part of the network portion. If it is 0, it is part of the host portion
- Calculating Network Address: To calculate the network address, the subnet mask is bitwise ANDed with the IP address. The result is the network address
- Determining Host Range: The host range is determined by the number of bits in the host portion of the subnet mask. The host range starts from the network address + 1 and ends at the broadcast address - 1
- Default Subnet Masks: There are default subnet masks assigned to different IP address classes. Class A uses a default subnet mask of 255.0.0.0, Class B uses 255.255.0.0, and Class C uses 255.255.255.0
- Variable Length Subnet Masking (VLSM): VLSM allows for the use of different subnet masks within the same network, allowing for more efficient use of IP addresses
- CIDR Notation: CIDR notation is used to represent subnet masks in a more concise way. It consists of the IP address followed by a slash (/) and the number of network bits in the subnet mask. For example, 192.168.0.0/24 represents a Class C network with a subnet mask of 255.255.255.0
- Subnetting: Subnetting is the process of dividing a network into smaller subnetworks using subnet masks. It helps in efficient utilization of IP addresses and improves network performance and security
Underlying Motivations
What the Interviewer is trying to find out about you and your experiences through this question
- Technical knowledge: Assessing the candidate's understanding of subnetting and network addressing
- Problem-solving skills: Evaluating the candidate's ability to apply subnet masks to divide networks and allocate IP addresses
- Communication skills: Determining if the candidate can explain complex networking concepts in a clear and concise manner
Potential Minefields
How to avoid some common minefields when answering this question in order to not raise any red flags
- Lack of understanding: Providing a vague or incorrect definition of a subnet mask
- Confusion with IP address: Mixing up the subnet mask with an IP address or not understanding the relationship between the two
- Inability to explain usage: Failing to explain how a subnet mask is used to determine network and host portions of an IP address
- Lack of practical knowledge: Not being able to provide examples or scenarios where subnet masks are used in network engineering
- Not considering CIDR notation: Neglecting to mention the use of CIDR notation to represent subnet masks