Basically every group of 4 binary digits can be used to make a hexidecimal digit. So first break up your binary number into groups of 4 digits starting at the dot and working outward toward the left and right:
0001__1010__1011__1011 . 0101__1000
Notice that I padded the left and the right side of the number with zeros to get a nice round 4 binary digits. That does not change the value of the binary number. It is very important to do this padding with the fractional part to the right of the dot, since there is a big difference between 1000 (with padding) and 1 (misleading appearance without padding).
Convert each group of 4 from binary to decimal and then from decimal to hexidecimal:
0000 == 0(dec) == 0(hex)
0001 == 1(dec) == 1(hex)
0010 == 2(dec) == 2(hex)
...
...
1001 == 9(dec) == 9(hex)
1010 == 10(dec) == A(hex)
1011 == 11(dec) == B(hex)
1100 == 12(dec) == C(hex)
1101 == 13(dec) == D(hex)
1110 == 14(dec) == E(hex)
1111 == 15(dec) == F(hex)
So for your binary number grouped for hexidecimal conversion:
0001__1010__1011__1011 . 0101__1000
the decimal-equivalent groupings would be:
1__10__11__11 . 5__8
so the hexidecimal would be:
1ABB.58
ASIDE TO CHIPZ: the fractional notation using a dot works just as well for arithmetic of any base as it works for base 10. However, when the dot is used with binary digits it is called a "binary point," not a "decimal point".
Now answer this bonus question for a free WHATZIT(TM): What is the dot called when it is used for fractional notation with hexidecimal digits?
And a bonus bonus question, this one for a free WHATZIT(TM) REMOVER: Why do computer programmers and engineers represent computer-based "floating point numbers" as 8- or 16-digit hexidecimal integers instead of using "hexidecimal point" notation?