Cisco CCNA Exam: Subnetting for a National Retail Network

# Subnetting for a National Retail Network

### Question

A national retail chain needs to design an IP addressing scheme to support a nationwide network. The company needs a minimum of 300 sub-networks and a maximum of 50 host addresses per subnet. Working with only one Class B address, which of the following subnet masks will support an appropriate addressing scheme? (Choose two.)

### Explanations

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A. B. C. D. E. F.

BE

Subnetting is used to break the network into smaller more efficient subnets to prevent excessive rates of Ethernet packet collision in a large network. Such subnets can be arranged hierarchically, with the organization's network address space (see also Autonomous System) partitioned into a tree-like structure.

Routers are used to manage traffic and constitute borders between subnets. A routing prefix is the sequence of leading bits of an IP address that precede the portion of the address used as host identifier. In IPv4 networks, the routing prefix is often expressed as a "subnet mask", which is a bit mask covering the number of bits used in the prefix. An IPv4 subnet mask is frequently expressed in quad-dotted decimal representation, e.g., 255.255.255.0 is the subnet mask for the

192.168.1.0 network with a 24-bit routing prefix (192.168.1.0/24).

To support a minimum of 300 sub-networks and a maximum of 50 hosts per subnet using only one Class B address, we need to use subnetting. Subnetting is the process of dividing a network into smaller sub-networks, which helps to conserve IP addresses and manage network traffic.

We can determine the number of bits needed for the subnet mask by calculating the total number of required subnets and the total number of required hosts per subnet. In this case, we need a minimum of 300 sub-networks and a maximum of 50 hosts per subnet.

To determine the number of bits needed for the subnet mask, we can use the following formula:

2^n >= Number of subnets required

where n is the number of bits in the subnet mask.

For example, if we need 300 subnets, we can calculate the number of bits required as follows:

2^n >= 300 n >= log2(300) n >= 8.228

Since we cannot use a fractional number of bits in the subnet mask, we need to round up to the nearest whole number of bits. Therefore, we need at least 9 bits for the subnet mask to support a minimum of 300 sub-networks.

To determine the number of bits needed for the host portion of the IP address, we can use the following formula:

2^n - 2 >= Number of hosts required

where n is the number of bits in the host portion of the IP address.

For example, if we need a maximum of 50 hosts per subnet, we can calculate the number of bits required as follows:

2^n - 2 >= 50 n >= log2(52) n >= 5.7

Again, we need to round up to the nearest whole number of bits. Therefore, we need at least 6 bits for the host portion of the IP address to support a maximum of 50 hosts per subnet.

So, the subnet mask that can support the required addressing scheme must have 9 bits for the subnet portion and 6 bits for the host portion.

Now, we can determine the subnet mask by converting the number of bits to decimal form. The first 9 bits will be set to 1, and the remaining 7 bits will be set to 0, resulting in a subnet mask of 255.255.254.0 or /23.

Therefore, the two subnet masks that can support the required addressing scheme are:

B. 255.255.255.128 (/25) E. 255.255.254.0 (/23)

Option B allows for a maximum of 126 subnets with 128 hosts per subnet. Option E allows for a maximum of 512 subnets with 510 hosts per subnet.