CCIE – by Jeff Doyle, First Volume Summary
IP Header Fields:
- Because the largest decimal number that can be described with 16 bits is 65,535, the maximum possible size of an IP packet is 65,535 octets.
- Don’t Fragment (DF) bit. When the DF bit is set to one, a router cannot fragment the packet. If the packet cannot be forwarded without fragmenting, the router drops the packet and sends an error message to the source. This function enables the testing of MTUs in an internetwork. Used with extended Ping.
- MF Bit: When a router fragments a packet, it sets the MF bit to one in all but the last fragment so that the receiver knows to keep expecting fragments until it encounters a fragment with MF = 0.
- The recommended default TTL is 64, although values such as 15 and 32 are not uncommon.
- Trace: if the router is told to trace the route to a host address such as 10.11.12.13, the router will send three packets with the TTL set to one; the first router will decrement it to zero, drop the packets, and send back error messages to the source. By reading the source address of the error messages, the first router on the path is now known. The next three packets will be sent with a TTL of two. The first router decrements to one, the second to zero, and an error message is received from the second router. The third set has a TTL of three, and so forth, until the destination is found. All routers along the internetwork path will have identified themselves.
- The First Octet Rule: Class B addresses are for medium-size internetworks. The first two octets are the network portion , and the last two octets are the host portion. There are 216 or 65,536 available numbers in the network part and an equal number in the host part.
- For class A addresses, the first bit of the first octet— that is, the left-most bit of the entire 32-bit string
- Class B addresses always have their left-most bit set to one and the second bit set to zero.
- In class C addresses, the first two bits are set to one, and the third bit is set to zero. The result is a first octet range of 192 through 223
- Likewise, classful routing protocols cannot differentiate a broadcast on the all-ones subnet from an all-subnets broadcast address For example, the all-ones subnet would be 172.21.255.0. For that subnet, the all-hosts broadcast address would be 172.21.255.255, but that is also the broadcast for all hosts on all subnets of major network 172.21.0.0
- Although the address mask must be specified to Cisco routers in dotted decimal, using the command, the mask may be displayed by various show commands in any of the three formats by using the command ip netmask-format [dec|hex|bit] in line configuration mode.
- given a class A address of 10.0.0.0, a subnet mask of 10.0.0.0/16 (255.255.0.0) means that the 8-bit subnet space will yield 28 – 2 = 254 available subnets and 216 – 2 = 65,534 host addresses available on each of those subnets.