qr code c# tutorial 22: Access Control Lists in Objective-C

Printer QR in Objective-C 22: Access Control Lists

22: Access Control Lists
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Third byte: 255 240 (third subnet byte value) = 15 (wildcard mask value) Fourth byte: 255 0 (fourth subnet byte value) = 255 (wildcard mask value)
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As you can see, this results in a wildcard mask of 0015240 This simple trick makes converting subnet masks to wildcard masks (and vice versa) very easy
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Wildcard masks are used to match against bits in a packet A 0 in a bit position means match, and a 1 means ignore If you want to match against a subnet, invert the corresponding subnet
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maskThe trick is to subtract each octet in the subnet mask from 255, resulting in the wildcard mask Don t be surprised if you have to use this trick for a handful of ACL questions on the exam
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Special Wildcard Masks
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Two special types of wildcard masks exist: 0000 and 255255255255 A wildcard mask of 0000 tells the IOS that all 32 bits of the address in the ACL statement must match those found in the IP packet in order for the IOS to execute the action for the statement A 0000 wildcard mask is called a host mask Here s a simple example of this information in an ACL statement: 19216811 0000 This statement tells the IOS to look for the exact same IP address (19216811) in the IP packet If the IOS doesn t find a match, the IOS will go to the next ACL statement If you configure 19216811 0000, the IOS will covert this to the following syntax: host 1721611 (note the keyword host that precedes the IP address) A wildcard mask of 255255255255 tells the router the exact opposite of a 0000 mask In this mask, all of the bit values are 1s, which tells the IOS that it doesn t matter what is in the packet that it is comparing to the ACL statement any address will match Typically, you would record this as an IP address of 0000 and a wildcard mask of 255255255255, like this: 0000 255255255255 If you enter this, the IOS will convert the address and mask to the keyword any Actually, the IP address that you enter with this mask doesn t matter For instance, if you enter 19216811 255255255255, this still matches any IP address Remember that it s the wildcard mask that determines what bits in the IP address are interesting and should match
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Wildcard Mask Examples
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Since the concept of a wildcard mask can be confusing, let s look at some examples Table 22-4 shows some examples of addresses and wildcard masks
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Basic ACL Con guration
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TABLE 22-4
IP Address
0000 1721611 1721610 1721620 1721600
Wildcard Mask
255255255255 0000 000255 001255 00255255
Matches
Match on any address (keyword any in an ACL statement) Match only if the address is 1721611 (preceded by the keyword host) Match only on packets that are in 1721610/24 (1721610 172161255) Match only on packets that are in 1721620/23 (1721620 172163255) Match only on packets that are in 1721600/16 (1721600 17216255255)
Wildcard Mask Examples
Some wildcard masks can be confusing, such as 001255 For masks like these, it s sometimes easier to look at them from a subnet mask perspective In this example, the corresponding subnet mask would be 2552552540 You can use a simple trick here by subtracting the wildcard mask from a local broadcast address (255255255255) to come up with the correct wildcard mask Based on this little trick, it s easier to see why row 4 of Table 22-4 matches on a range of addresses from 1721620 to 172163255
Be familiar with how wildcard masks work, as well as the special
notation Cisco uses for a match on all devices or a speci c host, as shown in Table 22-4
Activating an ACL
Once you have built your IP ACL, it will do nothing until you apply it to a process in the IOS This chapter focuses primarily on filtering traffic through interfaces Therefore, to have the IOS filter traffic between interfaces, you must enter the appropriate interface or interfaces and activate your ACL Here s the command to activate an ACL on an interface:
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