Cisco CCNA mock exam questions sample test – Question 35

.What is the valid host address range for the subnet 172.25.4.0 /23?

A. 172.25.4.1 to 172.25.5.254
B. 172.25.4.10 to 172.25.5.210
C. 172.25.4.35 to 172.25.5.64
D. 172.25.4.21 to 172.25.5.56

Correct Answer: A

Explanation:
For the subnet 172.25.4.0, the valid host range will start at 172.25.4.1 and end at 172.25.5.254.
To determine the valid range of addresses in a subnet, one must determine the subnet number or network ID and the broadcast address of the subnet and all valid addresses will lie
within those boundaries.
In this case:
Network address: 172.25.0.0
Subnet mask in decimal: 255.255.254.0 (/23 indicates 23 bit in the mask)
Subnet mask in binary: 11111111.11111111.11111110.00000000
The formulas to calculate the number of subnets and hosts are:
Number of subnets = 2number-of-subnet-bits
Number of hosts per subnet = 2number-of-host-bits – 2
In this scenario:
Number of subnet bits: 7 (the binary 1s in the third octet of the subnet mask)
Number of subnets: 27 = 128
Number of host bits: 9 (the binary 0s in the subnet mask)
Number of hosts: 29 – 2 = 510
These formulas are useful when determining if a subnet mask/network ID combination will support a given number of hosts.
To determine the boundaries of each of the 128 subnets that this mask will yield, you should utilize a concept called the interval or block size. This number helps to identify the
distance between network IDs. Determining the network IDs allows the identification of the broadcast address for each subnet, because the broadcast address for any particular
subnet will always be the last address before the next network ID. The interval is determined by the value of the far right-hand bit in the mask, which is 2 in this case. Then it is applied
to the octet where the mask ends. In this case, the first 4 network IDs are:
172.25.0.0
172.25.2.0
172.25.4.0
172.25.6.0
…incrementing by two at each point
Therefore, the valid addresses in the 172.25.4.0 network are framed by the two addresses that cannot be used: 172.25.4.0 (network ID) and 172.25.5.255 (broadcast address, or the
last address before the next network ID). The addresses within these boundaries are 172.25.4.1 to 172.25.5.254.
For subnet 172.25.0.0, the valid host range will run from 172.25.0.1 to 172.25.1.254. The broadcast address for subnet 172.25.0.0 will be 172.25.1.255.
For subnet 172.25.2.0, the valid host range will run from 172.25.2.1 to 172.25.3.254. The broadcast address for subnet 172.25.2.0 is 172.25.3.255.
For the subnet 172.25.4.0, the valid host range will run from 172.25.4.1 to 172.25.5.254. The broadcast address for subnet 172.25.4.0 is 172.25.5.255.
Always remember that the first address of each subnet is the network ID, and as such cannot be used as a host or router IP address. Also, the last address of each subnet is the
broadcast address for the subnet, and as such cannot be used as a host or router IP address.
Objective:
Network Fundamentals
Sub-Objective:
Compare and contrast IPv4 address types
References:
Cisco > Support > Technology Support > IP > IP Routing > Design > Design Technotes > IP Addressing and Subnetting for New Users > Document ID: 13788 > Understanding IP
Addresses