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An access list is an ordered list of statements denying or permitting packets based on matching criteria contained within the packet Let's examine this definition in greater detail An access list is an ordered list In other words, the order in which the statements are created in an access list is very important One of the most common mistakes made when creating access lists is entering the access list statements in an incorrect order Access list statements can either permit or deny packets Additionally, it should be pointed out now that there is always an implicit "Deny All" statement at the end of a Cisco access list A packet that is not explicitly permitted will be rejected by the implicit Deny All statement at the end of the access list Another common mistake when creating access lists is forgetting this fact The criteria used to permit or deny packets is based solely on information contained within the packet itself Usually, this information is restricted to information contained within the layer 3 or layer 4 header Therefore, with few exceptions, access lists cannot use information above layer 4 to filter packets Although application-specific commands may be contained in the data portion of the packet, an access list is not capable of filtering based on this information For example, an access list is not capable of filtering specific FTP commands The one caveat to this is the use of Context-Based Access Control (CBAC), which has the capability to filter packets based on wellknown application layer information CBAC will be covered in detail in 8, "Advanced Cisco Router Security Features" The above discussion is not meant to imply that the only function of access lists is packet filtering This is most certainly not the case Cisco access lists are used for many purposes other than packet filtering In fact, anytime a list of permitted addresses and/or protocols is needed to define which packets are to be included in a process, an access list is used Some of the other situations in which access lists can be used are as follows: Dial on Demand: Access lists are used to define which packets are permitted to cause a dial connection to occur This is commonly called defining interesting packets Queuing Features: Access lists define which types of packets are allocated to different types of outgoing queues so that some classes of packets are given priority over others Routing Update Filters: Access lists prevent certain network routes from being advertised or accepted by routing protocols Router Access: Access lists control telnet and SNMP access to the router itself This is in contrast to filtering packets that pass through the router As can be seen from this list, access lists play an integral role in many features used in Cisco routers, but not all of these features are related to security In addition to examining the use of access lists for packet filtering, we will briefly examine the use of access lists to control access to.

Class C addresses have a subnet mask of 2552552550, which means that the first three octets are the network ID and the last octet is the host ID Having only one octet as the host ID means that a class C address can support only 254 hosts (256 2) on the network You can identify a class C address because Class C addresses have an it has a value for the first octet that ranges IP address in which the value of the first between 192 and 223 For example, an IP octet is between 192 and 223 In addition, address of 2024586 is a class C address class C addresses have a default subnet because 202 falls between 192 and 223 You mask of 2552552550 also know that this system has a subnet mask of 2552552550 because it is a class C address

Class D addresses are used for special types of applications on the network known as multicasting applications Multicasting applications send data to a number of systems at the same time by sending data to the multicast address, and anyone who has registered with that address will receive the data A multicast address is what class D addresses are used for, so you will not be assigning them specifically to hosts on the network for normal network communication Class D addresses have a value on the first octet that ranges from 224 to 239 With that many ranges, class D has the potential for 268,435,456 unique multicast groups that users can subscribe to from a multicast application

If you are running a version of Cisco IOS earlier than version 11, you will see one unnerving feature of the 2500 series if you attach a terminal to the console port during boot-up The ROM IOS checks the configuration file and will not recognize most of the commands This results in many error messages being reported to the screen This is normal operation When the IOS in flash memory loads, normally no error messages are displayed RAM Random access memory (RAM) is split by the IOS into shared and main memory Main memory is used to store router configuration and IOS data structures relevant to the protocol being routed For IP, main memory is used for such things as holding routing tables and ARP tables; for IPX, main memory holds SAP and other tables (These terms are explained later) Shared memory buffers packets waiting to be processed This type of memory is only used by 4000- and 2500-series routers The 7000 routers have a switch processor that controls the flow of packets through the router Flash Memory Flash memory holds the current version of IOS running on the router Flash memory is erasable memory that can be overwritten with newer versions of the IOS unlike ROM, which is located in physical chips that cannot have their contents overwritten NVRAM Nonvolatile RAM (NVRAM) does not lose its contents when the router is switched off NVRAM holds the router configuration Booting a Router Routers boot up in a similar fashion to PCs; the procedure is as follows: 1Load bootstrap program from ROM 2Load operating system (IOS, the Internetwork Operating System) from flash memory 3Find and load configuration file in NVRAM or on a prespecified network server If no configuration file exists, the router enters setup mode

4:

the router and their use in filtering routing protocol updates Many sources also provide information on these and other uses of access lists on the Cisco web site The remainder of this chapter will concentrate on the use of access lists for packet filtering

The funny thing about class E addresses is that they were designed for experimental purposes only, so you will never see a class E address on a network Class E addresses have a first octet with a value that falls in the range of 240 247 Now that you are familiar with the different class addresses, take a look at Table 4-5, which summarizes the address classes Be sure to know them for the exam

tabLe 4-5

This section explains connecting a terminal to a router, understanding the Cisco user interface, and executing basic Cisco commands

255000 25525500 2552552550