The TCP/IP Protocol Layers
The IP Packet Header
IP Addresses
ARP
ICMP
The Host-to-Host Layer
Header Length is a four-bit field that tells, as the name implies, the length of the IP header. The reason this field is included is that the Options field(described later in this section) can vary in size. The minimum length of the IP header is 20 octets, and the options may increase this size up to a maximum of 24 octets. This field describes the length of the header in terms of 32-bit words-five for the minimum 160-bit size and six for the maximum
The DF bit can be used in troubleshooting to determine a path's MTU
Flags is a three-bit field in which the first bit is unused. The second is the 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. The DF bit can be set using the Extended Ping utility on Cisco router.
An important distinction to remember when working with IP addresses is that dotted decimal is just an easy way for humans to read and write IP addresses. Always remember that the router is not reading an address in terms of four octets; rather, the router sees a 32-bit binary string. Many pitfalls can be avoided by keeping this fact firmly in mind
So far IP addressing doesn't seem so difficult. A router or host could easily determine the network part of an IP address by using the first octet rule. If the first bit is 0,then read the first eight bits to find the network address. If the first two bits are 10, then read the first 16 bits; and if the first three bits are 110, then read 24 bits in to get the network address.
Unfortunately, things are not that easy
Most of the addresses used in this book are from this reserved pool, described in RFC 1918. Reserved addresses are:10.0.0.0-10.255.255.255, 172.16.0.0-172.31.255.255 and 192.168.0.0-192.168.255.255
To configure a router to display its masks in bitcount format, use:
Gladys(config)# line vty 0 4
Gladys(config-line)# ip netmask-format bit
To change the ARP timeout to 30 minutes(1,800 seconds):
Martha(config)# interface ethernet 0
Martha(config-if)# arp timeout 1800
ARP entries may also be permanently placed in the table. To statically map 172.21.5.131 to hardware address 0000.00a4.b74c, with a SNAP encapsulation type, use the following:
Martha(config)# arp 172.21.5.131 0000.00a4.b74c snap
Proxy ARP is enabled by default on Cisco routers and may be disabled on a per interface basis with the command "no ip proxy-arp"
RARP is being largely supplanted by Bootstrap Protocol(BOOTP) and its extension Dynamic Host Configuration Protocol(DHCP), both of which can provide more information than the IP address, and which, unlike RARP, can be routed off the local data link
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