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As of December 2018 the NTP pool consisted of around 7,700 active time servers on IPv4.
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ip-neighbour(8) [centos man page]

IP-NEIGHBOUR(8) 						       Linux							   IP-NEIGHBOUR(8)

NAME
ip-neighbour - neighbour/arp tables management. SYNOPSIS
ip [ OPTIONS ] neigh { COMMAND | help } ip neigh { add | del | change | replace } { ADDR [ lladdr LLADDR ] [ nud { permanent | noarp | stale | reachable } ] | proxy ADDR } [ dev DEV ] ip neigh { show | flush } [ proxy ] [ to PREFIX ] [ dev DEV ] [ nud STATE ] DESCRIPTION
The ip neigh command manipulates neighbour objects that establish bindings between protocol addresses and link layer addresses for hosts sharing the same link. Neighbour entries are organized into tables. The IPv4 neighbour table is also known by another name - the ARP ta- ble. The corresponding commands display neighbour bindings and their properties, add new neighbour entries and delete old ones. ip neighbour add add a new neighbour entry ip neighbour change change an existing entry ip neighbour replace add a new entry or change an existing one These commands create new neighbour records or update existing ones. to ADDRESS (default) the protocol address of the neighbour. It is either an IPv4 or IPv6 address. dev NAME the interface to which this neighbour is attached. lladdr LLADDRESS the link layer address of the neighbour. LLADDRESS can also be null. nud NUD_STATE the state of the neighbour entry. nud is an abbreviation for 'Neighbour Unreachability Detection'. The state can take one of the following values: permanent the neighbour entry is valid forever and can be only be removed administratively. noarp the neighbour entry is valid. No attempts to validate this entry will be made but it can be removed when its lifetime expires. reachable the neighbour entry is valid until the reachability timeout expires. stale the neighbour entry is valid but suspicious. This option to ip neigh does not change the neighbour state if it was valid and the address is not changed by this command. ip neighbour delete - delete a neighbour entry This command invalidates a neighbour entry. The arguments are the same as with ip neigh add, except that lladdr and nud are ignored. Warning: Attempts to delete or manually change a noarp entry created by the kernel may result in unpredictable behaviour. Particularly, the kernel may try to resolve this address even on a NOARP interface or if the address is multicast or broadcast. ip neighbour show - list neighbour entries This command displays neighbour tables. to ADDRESS (default) the prefix selecting the neighbours to list. dev NAME only list the neighbours attached to this device. proxy list neighbour proxies. unused only list neighbours which are not currently in use. nud NUD_STATE only list neighbour entries in this state. NUD_STATE takes values listed below or the special value all which means all states. This option may occur more than once. If this option is absent, ip lists all entries except for none and noarp. ip neighbour flush - flush neighbour entries This command flushes neighbour tables, selecting entries to flush by some criteria. This command has the same arguments as show. The differences are that it does not run when no arguments are given, and that the default neighbour states to be flushed do not include permanent and noarp. With the -statistics option, the command becomes verbose. It prints out the number of deleted neighbours and the number of rounds made to flush the neighbour table. If the option is given twice, ip neigh flush also dumps all the deleted neighbours. EXAMPLES
ip neighbour Shows the current neighbour table in kernel. ip neigh flush dev eth0 Removes entries in the neighbour table on device eth0. SEE ALSO
ip(8) AUTHOR
Original Manpage by Michail Litvak <mci@owl.openwall.com> iproute2 20 Dec 2011 IP-NEIGHBOUR(8)

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