10.4.2 Calculating Addresses: Case 2 Page 1:

In Case 2, the challenge is to subnet this internetwork while limiting the number of wasted hosts and subnets.

The figure shows 5 different subnets, each with different host requirements. The given IP address is 192.168.1.0/24.

The host requirements are:

• NetworkA - 14 hosts

• NetworkB - 28 hosts

• NetworkC - 2 hosts

• NetworkD - 7 hosts

• NetworkE - 28 hosts

As we did with Case 1, we begin the process by subnetting for the largest host requirement first. In this case, the largest requirements are for NetworkB and NetworkE, each with 28 hosts.

We apply the formula: usable hosts = 2^n - 2. For networks B and E, 5 bits are borrowed from the host portion and the calculation is 2^5 = 32 - 2. Only 30 usable host addresses are available due to the 2 reserved addresses. Borrowing 5 bits meets the requirement but gives little room for growth.

So you may consider borrowing 3 bits for subnets leaving 5 bits for the hosts. This allows 8 subnets with 30 hosts each.

We allocate addresses for networks B and E first:

Network B will use Subnet 0: 192.168.1.0/27

host address range 1 to 30

Network E will use Subnet 1: 192.168.1.32/27

host address range 33 to 62

The next largest host requirement is NetworkA, followed by NetworkD.

Borrowing another bit and subnetting the network address 192.168.1.64 yields a host range of:

Network A will use Subnet 0: 192.168.1.64/28

host address range 65 to 78

Network D will use Subnet 1: 192.168.1.80/28

host address range 81 to 94

This allocation supports 14 hosts on each subnet and satisfies the requirement.

Network C has only two hosts. Two bits are borrowed to meet this requirement.

Starting from 192.168.1.96 and borrowing 2 more bits results in subnet 192.168.1.96/30.

Network C will use Subnet 1: 192.168.1.96/30

host address range 97 to 98

In Case 2, we have met all requirements without wasting many potential subnets and available addresses.

In this case, bits were borrowed from addresses that had already been subnetted. As you will recall from a previous section, this method is known as Variable Length Subnet Masking, or VLSM.

10.4.2 - Calculating Addresses: Case 2 The diagram depicts calculating addresses for host requirements.  Network Topology:  The diagram shows five different subnets, each with different host requirements.  - Network A LAN with 14 hosts is connected to router R1's FA0/0 Ethernet interface.  - Network B LAN with 28 hosts is connected to router R1's FA0/1 Ethernet interface.  - Network D LAN with 7 hosts is connected to router R2's FA0/0 Ethernet interface.  - Network E LAN with 28 hosts is connected to router R2's FA0/1 Ethernet interface.  - Network C WAN interconnects router R1 and R2.

10.5 Device Interconnections

10.5.1 Device Interfaces Page 1:

It is important to understand that Cisco devices, routers, and switches have several types of interfaces associated with them. You have worked with these interfaces in the labs. These interfaces, also commonly called ports, are where cables are connected to the device. See the figure for some example interfaces.

LAN Interfaces - Ethernet

The Ethernet interface is used for connecting cables that terminate with LAN devices such as computers and switches. This interface can also be used to connect routers to each other. This use will be covered in more detail in future courses.

Several conventions for naming Ethernet interfaces are popular, including AUI (older Cisco devices using a transceiver), Ethernet, FastEthernet and Fa 0/0. The name used depends on the type and model of the device.

WAN Interfaces - Serial

Serial WAN interfaces are used for connecting WAN devices to the CSU/DSU. A CSU/DSU is a device used to make the physical connection between data networks and WAN provider's circuits.

Serial interfaces between routers will also be used in our labs as part of various courses. For lab purposes, we will make a back-to-back connection between two routers using serial cables, and set a clock rate on one of the interfaces.

You may also need to configure other Data Link and Physical layer parameters on a router. To establish communication with a router via a console on a remote WAN, a WAN interface is assigned a Layer 3 address (IPv4 address).

Console Interface

The console interface is the primary interface for initial configuration of a Cisco router or switch. It is also an important means of troubleshooting. It is important to note that with physical access to the router's console interface, an unauthorized person can interrupt or compromise network traffic. Physical security of network devices is extremely important.

Auxiliary (AUX) Interface

This interface is used for remote management of the router. Typically, a modem is connected to the AUX interface for dial-in access. From a security standpoint, enabling the option to connect remotely to a network device carries with it the responsibility of maintaining vigilant device management.

10.5.1 - Device Interfaces The diagram depicts example device interfaces on the router back panel  of a Cisco 1841 router. The highlighted interfaces include:  - Console interface (one integrated)  - Serial interfaces (two smart serial interfaces on a WIC2T modular card)  - FastEthernet interfaces (two integrated)  - Auxiliary Interface (one integrated)