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Windows Vista Security: Praxisorientierte Sicherheit für Profis by Marcus Nasarek

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Networking with TCP/IP
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493
Using Private IPv4 Addresses and Networking Protocols
Some IP addresses designated for Class A, B, and C networks are defined as public
and others are defined as private. Public IP addresses are assigned by ISPs. ISPs
obtain their IP addresses from a local or national Internet registry. When you con-
nect directly to the Internet through dial-up or by connecting an ISP’s cable/DSL
modem directly to your computer, your computer uses a public IP address assigned
by your ISP. Not every computer that connects to the Internet needs its own IP
address, however. If it did, the IPv4 addressing scheme would have run out of new
addresses a long time ago. This is where private IP addresses come into the picture.
When you set up a network, you assign the computers on the network private IP
addresses. Private IP addresses are defined as follows:
Class A private IP addresses include the addresses from 10.0.0.0 through 10.255.
255.255.
Class B private IP addresses include the addresses from 172.16.0.0 through
172.31.255.255.
Class C private IP addresses include the addresses from 192.168.0.0 through
192.168.255.255.
Since private IP addresses are not routable to the Internet, your network can use the
same private IP addresses that other people are using with their networks. When a
computer is connected to a network that in turn connects to an ISP, your Ethernet
router is the device that is assigned a public IP address. Generally speaking, the
router’s public IP address is the address by which all the computers on your network
will be identified when they are accessing resources on the Internet.
Table 14-5. IPv4 packet information
+ Bits 0–3 4–7 8–15 16–18 15–31
0 Version Header length Type of service Total length
32 Identification Flags Fragment offset
64 Time to Live (TTL) Protocol Header checksum
96 Source address
information
128 Destination address
information
160 Optional information
160/152+ Data transmitted

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