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Congratulations on completing another skill.
Now it's time for a review in the form of a quiz.
So let's go ahead and jump in with our first question.
What is the correct ACL wildcard mask
to match the following routes using a single line?
So in order to match on each of these prefixes,
the correct answer would be 0.16.64.255.
Now, we might want to break that down in just a little bit more
detail.
So the first thing we have to look at here
is, unlike the examples that we did during our demonstrations,
I stepped this one up just a little bit.
And instead, we're looking at two octets that are actually
changing their value.
However, hopefully you can see that there is clearly
a pattern here.
So what we need to do is we need to take each of these values
and break them down into binary.
So I have that set up.
And first, we have our 71 and our 87.
So this would be dealing with the second octet.
And just like we've done before, we
want to look at what bits are different.
So in this case, it would be 0, because those are the same.
The first three bits are all the same.
It's the fourth bit that's different.
So that would be a 1 in our wildcard mask.
And then of course, the last four are also all zeros.
And if we translate this back to decimal,
this is going to be 16.
And that's where we get this value for our wildcard mask.
Likewise, for the third octet, we have a 39 and a 103.
If we break this down the same way--
once again, this would be a 0, this would be a 1,
and the rest of these would all be zeros.
This, of course, is 64, which gives us this value.
And that's how we figure it out.
Now, in a situation like this, we can often shortcut this.
Since this is only going to work when
we have a few bits, such as this one and this one that
are different, these are generally
going to be multiples of 2.
So 1, 2, 4, 8, 16, 32, 64, or 128.
So if we actually look at these values, what we could do is
we could just take the higher value--
in this case, 87--
and we could subtract the lower value--
in this case, 71.
And that would give us the 16, which again, correlates
to the 16 in the second octet.
And we could do the same thing here,
if we were to take 103 and subtract 39.
Again, this is 64, which is going to match this value,
because it's only that one bit that's
different between the values.
Now, it might not always break down quite that clean,
but it should be some combination
of the multiples of 2.
So for example, maybe if the difference came out to a 9,
we'd be pretty clear that that's probably an 8 and a 1,
since those are multiples of 2.
So just to make sure that's clear, whatever the answer is
to the difference in the two numbers,
it should be some combination of the powers of 2.
That might be easy to figure out, and it might not be.
If it's easy, like the 9 being an 8 and a 1.
Or in this case, it was flat out just a 16 and 64.
Well, then you can certainly do this sort of shorter way
of just doing the subtraction.
And if you do that, it's a little more complicated.
Then by all means, we can still do it this way.
It doesn't matter how complicated they are
if we break it down to binary.
But just to summarize, the correct wildcard mask
to match on these networks would be 0.16.64.255.
Next question.
Given the following prefix list, which
routes would be permitted?
IP prefix list PL_FILTER deny 10.1.0.0/16 greater than
or equal to 25.
IP prefix list PL_FILTER permit 10.1.0.0/16 less than or equal
to 24?
So of course, the big thing to watch on this one
is that we're looking for what's going to be permitted.
So we have to make sure that our answers are
permitted by the second statement,
yet not denied by the first.
So let's go down our list.
So first, answer one is 10.1.1.0/24.
Now for the deny statement, we do match the first 16 bits,
which is the requirement here.
But the mask has to be greater than or equal to 25,
which means that our usable range here is actually
25 to 32.
This does not fit into the range of 25 to 32,
so it would not be denied.
Moving on to the permit statement,
it's almost the same, except that it's
less than or equal to 24.
So in this case, the range is going to be 16 to 24
for the mask.
So all of that said, it means that answer one is, in fact,
going to be allowed based on the permit statement,
because the 24 is within the range,
and the first 16 bits match.
Answer number two.
The big problem we have with this answer is the /32.
That falls into this range up here.
And that's on the deny statement.
So even if it did match the permit statement, which we're
not even going to look at, it doesn't
matter because it's being denied first.
So two would be incorrect.
3 is 10.1.0.0/10.
Now, the problem with this one is once again
it doesn't fit into any of the ranges.
So the bit mask might be OK, but it
doesn't fit into the ranges for the subnet masks.
So again, 3 is going to be an incorrect answer.
Number 4, 10.1.64.0/18.
Now, this one might be a little bit confusing because of the 64
here.
But keep in mind, we're only telling it
to match on the first 16 bits.
Now, the 18 does not fall into the range
of the deny statement, which is, of course, the 25 to 32 again.
But it does fit within the 16 to 24.
And again, we're matching on those first 16 bits.
So that would make 4 a correct answer.
And number 5, once again, is a /26,
which falls into that range for the deny statement, 25 to 32.
So it's going to be denied.
Therefore, the correct answers to this question are 1 and 4.
So that concludes this quiz.
If you find that you were unable to answer
any of these questions, I would encourage
you to go back and review the referenced sources
on each question.
Otherwise, congratulations on completing
Explaining Route Filtering.
I hope this has been informative for you,
and I'd like to thank you for viewing.