Encode the message $[1,1,0,1,1,0,1]$ in BCH code based on the field $\mathbb F = \frac{\mathbb Z_{2}[x]}{x^4+x+1}$ So here's what I understand so far:
$\mathbb F = \frac{\mathbb Z_{2}[x]}{x^4+x+1} = GF(16)$
The code is written as $[x^{14},x^{13},x^{12},x^{11},x^{10},x^{9},x^{8}$ $|$ $x^{7},x^{6},x^{5},x^{4},x^{3},x^{2},x^{1},x^{0}]$
So we write the message $[1,1,0,1,1,0,1] = x^{14}+x^{13}+x^{11}+x^{10}+x^{8}$ and we get the first part of the code:
$[1,1,0,1,1,0,1$ $|$ $x^{7},x^{6},x^{5},x^{4},x^{3},x^{2},x^{1},x^{0}]$
We divide $x^{14}+x^{13}+x^{11}+x^{10}+x^{8}$ by $(x^8+x^7+x^6+x^4+1)$ and we get $(x^6+x^4+x^2+x)$ with remainder $(x^7+x^5+x^4+x^2+x)$
Taking the coefficients of the remainder we get the encoded BCH code
$[1,1,0,1,1,0,1$ $|$ $1,0,1,1,0,1,1,0]$ 
My question is: Why do we divide by $(x^8+x^7+x^6+x^4+1)$? Where did that come from?:

Yes, the question is somewhat incomplete (as pointed by Dilip Sarwate and Jyrki Lahtonen). 
This is a 2-error correcting (primitive narrow-sense) BCH code. The generator polynomial is the smallest binary polynomial that has $\alpha, \alpha^2, \alpha^3, \alpha^4$ as its roots. This is precisely the LCM of the minimal polynomials of those four roots. Also, the minimal polynomial of $\alpha, \alpha^2, \alpha^4$ are the same polynomial. 
