uCertify Chapter 8

On a network, which mask should you use on point-to-point WAN links to reduce the waste of IP addresses?

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

What is the highest usable address on the 172.16.1.0/24 network?

172.16.1.254; 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 value (172.16.1.0) and the all-ones value (172.16.1.255). The highest usable address, the last one before the all-ones value, is 172.16.1.254.

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?

172.16.112.0; A /25 subnet 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 bits 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 a network with a subnet of 172.16.17.0/22. Which of the following is a valid host address?

172.16.18.255 255.255.252.0; A Class B network ID having a /22 subnet 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. So, only 172.16.18.255 255.255.252.0 has the correct subnet mask listed, and 172.16.18.255 is a valid host.

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?

172.16.2.255/172.16.3.0; 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 will be the subnetwork of a host if it has the address 172.16.45.14/30?

172.16.45.12; A /30 subnet mask, regardless of the class of address, has 252 in the fourth octet. This means the block size is 4 and the subnets are 0, 4, 8, 12, 16, and so on. Address 172.16.45.14 is obviously in the 172.16.45.12 subnet.

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 the solution?

PAT; 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).

On which of the following devices NAT can be implemented?

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

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?

Router; The subnet mask is 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, with a broadcast address of 255. You need a router for these two hosts to communicate because they are in different subnets.

Subnet mask

A group of selected bits that identify a subnetwork within a TCP/IP network.

Host address

A logical address configured by an administrator or server on a device. It logically identifies this device on an internetwork.

Using the following figure, what would be the IP address of E0 if you were using the eighth subnet? The network ID is 192.168.10.0/28, and you need to use the last available IP address in the range. The zero subnet should not be considered valid for this question.

192.168.10.142; A /28 is a 255.255.255.240 subnet mask. To find the broadcast address of the eighth subnet, counting has to be done till the ninth subnet. Starting at 16 (remember, the question stated that we will not use subnet zero, so we start at 16, not 0), we get: 16, 32, 48, 64, 80, 96, 112, 128, 144. The eighth subnet is 128, and the next subnet is 144, so the broadcast address of the 128 subnet is 143. This makes the host range 129-142. Therefore, 142 is the last valid host and the IP address is 192.168.10.142.

Using the figure given below, what would be the IP address of E0 if you were using the first subnet? The network ID is 192.168.10.0/28, and you need to use the last available IP address in the range. The zero subnet should not be considered valid for this question.

192.168.10.30; A /28 is a 255.255.255.240 mask. Let's count to the second subnet (we need to find the broadcast address of the first subnet, so we need to count to the second subnet). Starting at 16 (remember, the question stated that we will not use subnet zero, so we start at 16, not 0), the first subnet is 16 and the next subnet is 32, so the broadcast address of the 16 subnet is 31. This makes the host range 17-30. 30 is the last valid host and the IP address is 192.168.10.30.

You have an interface on a router with the IP address of 192.168.192.10/29. What is the broadcast address of the hosts that will be used on this LAN?

192.168.192.15; A /29 (255.255.255.248) has a block size of 8 in the fourth octet. This means that the subnets are 0, 8, 16, 24, and so on. 192.168.192.10 is in the 8th subnet. The next subnet is 16, so 192.168.192.15 is the broadcast address.

What is the subnetwork address for a host with an IP address 200.10.5.68/28?

200.10.5.64; A /28 subnet mask is 255.255.255.240, which means that the block size is 16 in the fourth octet (0, 16, 32, 48, 64, 80, and so on). Therefore, the host is in the 64 subnet.

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?

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

The network address of 172.16.0.0/19 provides how many subnets and hosts?

8 subnets, 8,190 hosts each; 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. Therefore, there are 8 subnets, each with 8,190 hosts.

Network Address Translation (NAT)

A TCP/IP service that many routers, firewalls, and IP proxies can provide. NAT translates addresses that are legal for an inside network but illegal for a corresponding outside network into addresses that are legal for the outside network. NAT also resolves the outside addresses back to the inside addresses as return traffic for the originating device comes back from the outside network.

A network administrator is connecting two hosts directly through their Ethernet interfaces, as shown in the figure. Ping attempts between the hosts are unsuccessful. What can be done to provide connectivity between the hosts? Each correct answer represents a complete solution. Choose two.

A crossover cable should be used in place of the straight-through cable./The subnet masks should be set to 255.255.255.0.; First, if you have two hosts directly connected, as shown in the graphic, then you need a crossover cable. A straight-through cable won't work. Second, the hosts are in different subnets. The easy solution is just to set both masks to 255.255.255.0 (/24).

Which of the following is desirable while configuring the IP settings on a computer on one subnet so that it can communicate with a computer on another subnet?

Configure the computer with a default gateway that matches the IP address of its local router's interface. ; A computer should be configured with an IP address that is unique throughout the reachable inter-network. It should be configured with a subnet mask that matches those of all other devices on its local subnet, but not necessarily with one that matches the mask used on any other subnet. It should 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 he cannot get on the Internet. You verify the 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?

Incorrect gateway IP; The host ID of 10.0.37.144 with a 255.255.254.0 mask is in the 10.0.36.0 subnet. The third octet has a block size of 2, so the next subnet is 10.0.38.0, which makes 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 your interesting octet, which is the third octet in this question. 256 - 254 = 2. Your block size is 2.

Port Address Translation

It allows using the Transport layer to identify the hosts, which in turn allows using (theoretically) up to 65,000 hosts with one real IP address.

Dynamic NAT (DNAT)

It gives the ability to map an unregistered IP address to a registered IP address from a pool of registered IP addresses.

Overloading

It is a form of dynamic NAT that maps multiple unregistered IP addresses to a single registered IP address—many-to-one—by using different ports.

Static NAT (SNAT)

It is designed to allow one-to-one mapping between local and global addresses. It requires one real Internet IP address for every host on a network.

Port forwarding

It is translating the port number of a packet to a new destination.

Classless Inter-Domain Routing (CIDR)

It's basically the method that Internet service providers (ISPs) use to allocate a number of addresses to a company or a home connection.

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 a new router. Which of the following can you reference for assistance?

The default gateway settings on computers from each subnet that the old router inter-connected.; The best method is to check the configuration of devices that were using the old router as a gateway to the rest of the network. 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. Routers don't auto-configure themselves; we wouldn't want them to.

Subnetting

The process of dividing a single IP address range into multiple address ranges.