The following explanation below is quoted from the Web Application Hacker’s Handbook, written by
Dafydd Stuttard and Marcus Pinto.

—————————————-
“URLs are permitted to contain only the printable characters in the US-ASCII
character set — that is, those whose ASCII code is in the range 0x20 to 0x7e,
inclusive. Furthermore, several characters within this range are restricted because
they have special meaning within the URL scheme itself or within the HTTP
protocol.

The URL-encoding scheme is used to encode any problematic characters
within the extended ASCII character set so that they can be safely transported
over HTTP. The URL-encoded form of any character is the % prefi x followed by
the character’s two-digit ASCII code expressed in hexadecimal. Here are some
characters that are commonly URL-encoded:

     %3d — =
     %25 — %
     %20 — Space
     %0a — New line
     %00 — Null byte

A further encoding to be aware of is the + character, which represents a
URL-encoded space (in addition to the %20 representation of a space).

NOTE: For the purpose of attacking web applications, you should URLencode
any of the following characters when you insert them as data into an
HTTP request:

     space % ? & = ; + #

(Of course, you will often need to use these characters with their special
meaning when modifying a request — for example, to add a request parameter
to the query string. In this case, they should be used in their literal form.)”

—————————————-

The US ASCII Character Set

US ASCII, ANSI X3.4-1986 (ISO 646 International Reference Version)

Codes 0 through 31 and 127 (decimal) are unprintable control characters.
Code 32 (decimal) is a nonprinting spacing character. Codes 33 through 126 (decimal) are printable graphic characters.

Legend:
  Char     The printed representation of the character, if any
  Dec      The decimal code for the character
  Row/Col  The decimal row/column representation for the character
  Oct      The octal (base 8) code for the character
  Hex      The hexadecimal (base 16) code for the character

Char Dec Col/Row Oct Hex  Name and Description

       0  00/00   00  00  NUL  (Ctrl-@)  NULL
       1  00/01   01  01  SOH  (Ctrl-A)  START OF HEADING
       2  00/02   02  02  STX  (Ctrl-B)  START OF TEXT
       3  00/03   03  03  ETX  (Ctrl-C)  END OF TEXT
       4  00/04   04  04  EOT  (Ctrl-D)  END OF TRANSMISSION
       5  00/05   05  05  ENQ  (Ctrl-E)  ENQUIRY
       6  00/06   06  06  ACK  (Ctrl-F)  ACKNOWLEDGE
       7  00/07   07  07  BEL  (Ctrl-G)  BELL (Beep)
       8  00/08   10  08  BS   (Ctrl-H)  BACKSPACE
       9  00/09   11  09  HT   (Ctrl-I)  HORIZONTAL TAB
      10  00/10   12  0A  LF   (Ctrl-J)  LINE FEED
      11  00/11   13  0B  VT   (Ctrl-K)  VERTICAL TAB
      12  00/12   14  0C  FF   (Ctrl-L)  FORM FEED
      13  00/13   15  0D  CR   (Ctrl-M)  CARRIAGE RETURN
      14  00/14   16  0E  SO   (Ctrl-N)  SHIFT OUT
      15  00/15   17  0F  SI   (Ctrl-O)  SHIFT IN
      16  01/00   20  10  DLE  (Ctrl-P)  DATA LINK ESCAPE
      17  01/01   21  11  DC1  (Ctrl-Q)  DEVICE CONTROL 1 (XON)
      18  01/02   22  12  DC2  (Ctrl-R)  DEVICE CONTROL 2
      19  01/03   23  13  DC3  (Ctrl-S)  DEVICE CONTROL 3 (XOFF)
      20  01/04   24  14  DC4  (Ctrl-T)  DEVICE CONTROL 4
      21  01/05   25  15  NAK  (Ctrl-U)  NEGATIVE ACKNOWLEDGE
      22  01/06   26  16  SYN  (Ctrl-V)  SYNCHRONOUS IDLE
      23  01/07   27  17  ETB  (Ctrl-W)  END OF TRANSMISSION BLOCK
      24  01/08   30  18  CAN  (Ctrl-X)  CANCEL
      25  01/09   31  19  EM   (Ctrl-Y)  END OF MEDIUM
      26  01/10   32  1A  SUB  (Ctrl-Z)  SUBSTITUTE
      27  01/11   33  1B  ESC  (Ctrl-[)  ESCAPE
      28  01/12   34  1C  FS   (Ctrl-\)  FILE SEPARATOR
      29  01/13   35  1D  GS   (Ctrl-])  GROUP SEPARATOR
      30  01/14   36  1E  RS   (Ctrl-^)  RECORD SEPARATOR
      31  01/15   37  1F  US   (Ctrl-_)  UNIT SEPARATOR

( )   32  02/00   40  20                 SPACE
(!)   33  02/01   41  21                 EXCLAMATION MARK
(")   34  02/02   42  22                 QUOTATION MARK
(#)   35  02/03   43  23                 NUMBER SIGN
($)   36  02/04   44  24                 DOLLAR SIGN
(%)   37  02/05   45  25                 PERCENT SIGN
(&)   38  02/06   46  26                 AMPERSAND
(')   39  02/07   47  27                 APOSTROPHE
(()   40  02/08   50  28                 LEFT PARENTHESIS
())   41  02/09   51  29                 RIGHT PARENTHESIS
(*)   42  02/10   52  2A                 ASTERISK
(+)   43  02/11   53  2B                 PLUS SIGN
(,)   44  02/12   54  2C                 COMMA
(-)   45  02/13   55  2D                 HYPHEN, MINUS SIGN
(.)   46  02/14   56  2E                 PERIOD, FULL STOP
(/)   47  02/15   57  2F                 SOLIDUS, SLASH
(0)   48  03/00   60  30                 DIGIT ZERO
(1)   49  03/01   61  31                 DIGIT ONE
(2)   50  03/02   62  32                 DIGIT TWO
(3)   51  03/03   63  33                 DIGIT THREE
(4)   52  03/04   64  34                 DIGIT FOUR
(5)   53  03/05   65  35                 DIGIT FIVE
(6)   54  03/06   66  36                 DIGIT SIX
(7)   55  03/07   67  37                 DIGIT SEVEN
(8)   56  03/08   70  38                 DIGIT EIGHT
(9)   57  03/09   71  39                 DIGIT NINE
(:)   58  03/10   72  3A                 COLON
(;)   59  03/11   73  3B                 SEMICOLON
(<)   60  03/12   74  3C                 LESS-THAN SIGN, LEFT ANGLE BRACKET
(=)   61  03/13   75  3D                 EQUALS SIGN
(>)   62  03/14   76  3E                 GREATER-THAN SIGN, RIGHT ANGLE BRACKET
(?)   63  03/15   77  3F                 QUESTION MARK
(@)   64  04/00  100  40                 COMMERCIAL AT SIGN
(A)   65  04/01  101  41                 CAPITAL LETTER A
(B)   66  04/02  102  42                 CAPITAL LETTER B
(C)   67  04/03  103  43                 CAPITAL LETTER C
(D)   68  04/04  104  44                 CAPITAL LETTER D
(E)   69  04/05  105  45                 CAPITAL LETTER E
(F)   70  04/06  106  46                 CAPITAL LETTER F
(G)   71  04/07  107  47                 CAPITAL LETTER G
(H)   72  04/08  110  48                 CAPITAL LETTER H
(I)   73  04/09  111  49                 CAPITAL LETTER I
(J)   74  04/10  112  4A                 CAPITAL LETTER J
(K)   75  04/11  113  4B                 CAPITAL LETTER K
(L)   76  04/12  114  4C                 CAPITAL LETTER L
(M)   77  04/13  115  4D                 CAPITAL LETTER M
(N)   78  04/14  116  4E                 CAPITAL LETTER N
(O)   79  04/15  117  4F                 CAPITAL LETTER O
(P)   80  05/00  120  50                 CAPITAL LETTER P
(Q)   81  05/01  121  51                 CAPITAL LETTER Q
(R)   82  05/02  122  52                 CAPITAL LETTER R
(S)   83  05/03  123  53                 CAPITAL LETTER S
(T)   84  05/04  124  54                 CAPITAL LETTER T
(U)   85  05/05  125  55                 CAPITAL LETTER U
(V)   86  05/06  126  56                 CAPITAL LETTER V
(W)   87  05/07  127  57                 CAPITAL LETTER W
(X)   88  05/08  130  58                 CAPITAL LETTER X
(Y)   89  05/09  131  59                 CAPITAL LETTER Y
(Z)   90  05/10  132  5A                 CAPITAL LETTER Z
([)   91  05/11  133  5B                 LEFT SQUARE BRACKET
(\)   92  05/12  134  5C                 REVERSE SOLIDUS (BACKSLASH)
(])   93  05/13  135  5D                 RIGHT SQUARE BRACKET
(^)   94  05/14  136  5E                 CIRCUMFLEX ACCENT
(_)   95  05/15  137  5F                 LOW LINE, UNDERLINE
(`)   96  06/00  140  60                 GRAVE ACCENT
(a)   97  06/01  141  61                 SMALL LETTER a
(b)   98  06/02  142  62                 SMALL LETTER b
(c)   99  06/03  143  63                 SMALL LETTER c
(d)  100  06/04  144  64                 SMALL LETTER d
(e)  101  06/05  145  65                 SMALL LETTER e
(f)  102  06/06  146  66                 SMALL LETTER f
(g)  103  06/07  147  67                 SMALL LETTER g
(h)  104  06/08  150  68                 SMALL LETTER h
(i)  105  06/09  151  69                 SMALL LETTER i
(j)  106  06/10  152  6A                 SMALL LETTER j
(k)  107  06/11  153  6B                 SMALL LETTER k
(l)  108  06/12  154  6C                 SMALL LETTER l
(m)  109  06/13  155  6D                 SMALL LETTER m
(n)  110  06/14  156  6E                 SMALL LETTER n
(o)  111  06/15  157  6F                 SMALL LETTER o
(p)  112  07/00  160  70                 SMALL LETTER p
(q)  113  07/01  161  71                 SMALL LETTER q
(r)  114  07/02  162  72                 SMALL LETTER r
(s)  115  07/03  163  73                 SMALL LETTER s
(t)  116  07/04  164  74                 SMALL LETTER t
(u)  117  07/05  165  75                 SMALL LETTER u
(v)  118  07/06  166  76                 SMALL LETTER v
(w)  119  07/07  167  77                 SMALL LETTER w
(x)  120  07/08  170  78                 SMALL LETTER x
(y)  121  07/09  171  79                 SMALL LETTER y
(z)  122  07/10  172  7A                 SMALL LETTER z
({)  123  07/11  173  7B                 LEFT CURLY BRACKET, LEFT BRACE
(|)  124  07/12  174  7C                 VERTICAL LINE, VERTICAL BAR
(})  125  07/13  175  7D                 RIGHT CURLY BRACKET, RIGHT BRACE
(~)  126  07/14  176  7E                 TILDE

     127  07/15  177  7F   RUB (Ctrl-?)  RUBOUT (DELETE)