IP Address Prefixes

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IP Address Prefixes

• ACM programming practice problem.
• Ron Smith

2
CIDR Notation

• Block 190.101.118.99/22
• IP Address 190.101.118.99
• Network Prefix 22
• Represents ip addresses with prefixes
• 190.101.116.
• 190.101.117.
• 190.101.118.
• 190.101.119.

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Our Notation

• ip 190.101.118.99 n 22
• 190.101 . 116 .
• 190.101 . 117 .
• 190.101 . 118 .
• 190.101 . 119 .

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Critical Byte

• Given
• ip IP network (type list)
• n network prefix (type int)
• Compute
• cByte n/8 integer division
• common ip0cByte
• crit int(ipcByte)

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Splitting the critical byte

• Given
• Network prefix n 22
• Critical byte crit 118 (01110110)2
• 011101 10

fixed bits n 8
variable bits 8 - n 8
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Finding total, start

• Given
• Critical bit crit 118
• Variable bits vbits 2
• Find
• total 2vbits
• start crit crit total

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How to treat ip as an array

• def getitem(s,n,delim'.')
• s s.split(delim)
• s sn
• if type(s) list
• s '.'.join(s)
• return s

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How to handle dots.

• Dot after common
• preDot (cByte gt 0 and cBtye lt 4)'.'
• If cByte 0, common is empty No '.'
• If cByte 4, everything common No '.'
• Final dot
• postDot (cByte lt 4)'.'

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Ready to Code!