Chapter 8 IP Subnetting

Your router has the following IP address on Ethernet0: 172.16.2.1/23. Which of the following can be valid host IDs on the LAN interface attached to the router? (Choose two.) A. 172.16.0.5 B. 172.16.1.100 C. 172.16.1.198 D. 172.16.2.255 E. 172.16.3.0 F. 172.16.3.255

D, E. The router's IP address on the E0 interface is 172.16.2.1/23, which is a 255.255.254.0. This makes the third octet a block size of 2. The router's interface is in the 2.0 subnet, and the broadcast address is 3.255 because the next subnet is 4.0. The valid host range is 2.1 through 3.254. The router is using the first valid host address in the range.

What is the subnet for host ID 172.16.3.64/23?

A /23 is 255.255.254.0. The third octet is a block size of 2. 0, 2, 4. The host is in the 16.2.0 subnet; the broadcast address is 16.3.255.

On which of the following devices are you most likely to implement NAT? A. Hub B. Ethernet switch C. Router D. Bridge

C. Devices with layer 3 awareness, such as routers and firewalls, are the only ones that can manipulate the IP header in support of NAT.

What is the broadcast address of 192.168.192.10/29?

A /29 is 255.255.255.248. This is a block size of 8 in the fourth octet. 0, 8, 16. The host is in the 8 subnet, and broadcast is 15.

You have a Class B network and need 29 subnets. What is your mask?

A default Class B is 255.255.0.0. A Class B 255.255.255.0 mask is 256 subnets, each with 254 hosts. We need fewer subnets. If we use 255.255.240.0, this provides 16 subnets. Let's add one more subnet bit. 255.255.248.0. This is 5 bits of subnetting, which provides 32 subnets. This is our best answer, a /21.

If an ethernet port on a router was assigned an IP address of 172.16.112.1/25, what would be the subnet address of this host? A. 172.16.112.0 B. 172.16.0.0 C. 172.16.96.0 D. 172.16.255.0 E. 172.16.128.0

A. A /25 mask is 255.255.255.128. Used with a Class B network, the third and fourth octets are used for subnetting with a total of 9 subnet bits: 8 in the third octet and 1 bit in the fourth octet. Because there is only 1 bit in the fourth octet, the bit is either off or on - which is a value of 0 or 128. The host in the question is in the 0 subnet, which has a broadcast address of 127 because 128 is the next subnet.

You have an interface on a router with the IP address of 192.168.192.10/29. What is the broadcast address the hosts will use on this LAN? A. 192.168.192.15 B. 192.168.192.31 C. 192.168.192.63 D. 192.168.192.127 E. 192.168.192.255

A. A /29 (255.255.255.248) has a block size of 8 in the fourth octet. This means the subnets are 0, 8, 16, 24, and so on. 10 is in the 8 subnet. The next subnet is 16, so 15 is the broadcast address.

You have an interface on a router with the IP address of 192.168.192.10/29. Including the router interface, how many hosts can have IP addresses on the LAN attached to the router interface? A. 6 B. 8 C. 30 D. 62 E. 126

A. A /29 (255.255.255.248), regardless of the class address, has only 3 host bits. Six hosts is the maximum number of hosts on this LAN, including the router interface.

You have one IP address provided from your ISP with a /30 mask. However, you have 300 users that need to access the internet. What technology will you use to implement a solution? A. PAT B. VPN C. DNS D. LANs

A. Network Address Translation can allow up to 65,000 hosts to get onto the internet with one IP address by using Port Address Translation (PAT).

If you are forced to replace a router that has failed to the point that you are unable to access its current configuration to aid in setting up interface addresses on the new router, which of the following can you reference for assistance? A. The default gateway settings on computers from each subnet that the old router interconnected B. The router's configuration that was periodically cached on the DHCP server C. The router's configuration that was periodically cached on the DNS server D. The new router will auto-configure itself with the correct settings

A. The best method here is to check the configuration of devices that were using the old router as a gateway to the rest of the internetwork. Routers do not periodically cache their configurations to servers of any sort. You might have copied the old router's configuration to a TFTP server or the like, but failing that, you will have to rebuild the configuration from scratch, which might well be much more than interface addresses. Therefore, keeping a copy of the router's current configuration somewhere other than on the router is a wise choice. Router's don't auto-configure themselves; we wouldn't want them to.

What is the highest usable address on 172.16.1.0/24 network? A. 172.16.1.255 B. 172.16.1.254 C. 172.16.1.253 D. 172.16.1.23

B. A 24-bit mask, or prefix length, indicates that the entire fourth octet is used for host identification. In a special case, such as this, it is simpler to visualize the all-zeros values (172.16.1.0) and the all-ones value (172.6.1.255). The highest usable address, the last one before the all ones value, is 172.16.1.254.

You have a Class A host of 10.0.0.110/25. It needs to communicate to a host with an IP address of 10.0.0.210/25. Which of the following devices do you need to use in order for these hosts to communicate? A. A layer 2 switch B. Router C. DNS server D. Hub

B. Don't freak because this is a Class A. What is your subnet mask? 255.255.255.128. Regardless of the class of address, this is a block size of 128 in the fourth octet. The subnets are 0 and 128. The 0 subnet host range is 1-126, with a broadcast address of 127. The 128 subnet host range is 129-254, which a broadcast address of 255. You need a router for these two hosts to communicate because they are in different subnets.

When configuring the IP settings on a computer on one subnet to ensure that it can communicate with a computer on another subnet, which of the following is desirable? A. Configure the computer with the same default gateway as the other computer B. Configure the computer with the same subnet mask at the other computer C. Configure the computer with a default gateway that matches the IP address of the interface of the router that is attached to the same subnet as the computer D. Configure the computer with a subnet mask that matches the IP address of the interface of the router that is attached to the same subnet as the computer

C. A computer should be configured with an IP address that is unique throughout the reachable internetwork. It should be configured with a subnet mask that matches those of all other devices on its local subnet, but not necessarily one that matches the mask used on any other subnet. it should be also be configured with a default gateway that matches its local router's interface IP address.

You receive a call from a user who is complaining that they cannot get on the internet. You have them verify their IP address, mask, and default gateway. The IP address is 10.0.37.144, with a subnet mask of 255.255.254.0. The default gateway is 10.0.38.1. What is the problem? A. Incorrect DNS server address B. Invalid subnet mask C. Incorrect gateway IP D. IP address and mask not compatible

C. The host ID of 10.0.37.144 with a 255.255.254.0 mask is in the 10.0.36.0 subnet (yes, you need to be able to subnet in this exam!). Do not stress that this is a Class A; what we care about is that the third octet has a block size of 2, so the next subnet is 10.0.38.0, which masks the broadcast address 10.0.37.255. The default gateway address of 10.0.38.1 is not in the same subnet as the host. Even though this is a Class A address, you still should easily be able to subnet this because you look more at the subnet mask and find you interesting octet, which is the third octet in this question. 256-254=2. Your block size is 2.

What is the subnetwork address for a host with the IP address 200.10.5.68/28? A. 200.10.5.56 B. 200.10.5.32 C. 200.10.5.64 D. 200.10.5.0

C. This is a pretty simple question. A /28 is 255.255.255.240, which means that our block size is 16 in the fourth octet (0, 16, 32, 48, 64, 80, and so on). the host is in the 64 subnet.

What is the maximum number of IP addresses that can be assigned to hosts on a local subnet that uses the 255.255.255.224 subnet mask? A. 14 B. 15 C. 16 D. 30 E. 31 F. 62

D. A /27 (255.255.255.22) is 3 bits on and 5 bits off. This provides 8 subnets, each with 30 hosts. Does it matter if this mask is used with a Class A, B, or C network address? Not at all. The number of host bits would never change.

If a host on a network has the address 172.16.45.14/30, what is the subnetwork this host belongs to? A. 172.16.45.0 B. 172.16.45.4 C. 172.16.45.8 D. 172.16.45.12 E. 172.16.45.16

D. A /30, regardless of the class address, has a 252 in the fourth octet. This means we have a block size of 4 and our subnets are 0, 4, 8, 12, 16, and so on. Address 14 is obviously in the 12 subnet.

On a network, which mask should you use on point-to-point WAN links in order to reduce the waste of IP addresses? A. /27 B. /28 C. /29 D. /30 E. /31

D. A point-to-point link uses only two hosts. A /30, or 255.255.255.252, mask provides two hosts per subnet.

You have a network with a subnet of 172.16.17.0/22. Which of the following is a valid host address? A. 172.16.17.1 255.255.255.252 B. 172.16.0.1 255.255.240.0 C. 172.16.20.1 255.255.254.0 D. 172.16.16.1 255.255.255.240 E. 172.16.18.255 255.255.252.0 F. 172.16.0.1 255.255.255.0

E. A Class B network ID with a /22 mask is 255.255.252.0, with a block size of 4 in the third octet. The network address in the question is in subnet 172.16.16.0 with a broadcast address of 172.16.19.255. Only option E has the correct subnet mask listed, and 172.16.18.255 is a valid host.

The network address if 172.16.0.0/19 provides how many subnets and hosts? A. 7 subnets, 30 hosts each B. 7 subnets, 2046 hosts each C. 7 subnets, 8190 hosts each D. 8 subnets, 30 hosts each E. 8 subnets, 2046 hosts each F. 8 subnets, 8190 hosts each

F. A CIDR address of /19 is 255.255.224.0. This is a Class B address, so that is only 3 subnet bits, but it provides 13 host bits, or 8 subnets, each with 8190 hosts.