ip2hostname: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/ip2hostname - from package: flowscan

You may install the required package with command apt-get install {package}

Error in other languages:
ip2hostname: bevel nie gevind nie
ip2hostname: comandă negăsită
ip2hostname: comando não encontrado
ip2hostname: commande introuvable
ip2hostname: command not found
ip2hostname: käsku ei ole
ip2hostname: không tìm thấy lệnh
ip2hostname: komanda nerasta
ip2hostname: Kommando nicht gefunden.
ip2hostname: kommandot finns inte
ip2hostname: komut yok
ip2hostname: nie znaleziono polecenia
ip2hostname: níor aimsíodh an t-ordú
ip2hostname: no se encontró la orden
ip2hostname: no s'ha trobat l'ordre
ip2hostname: opdracht niet gevonden
ip2hostname: parancs nem található
ip2hostname: perintah tidak ditemukan
ip2hostname: príkaz nenájdený
ip2hostname: příkaz nenalezen
ip2hostname: команда не найдена
ip2hostname: командата не е открита
ip2hostname: 命令找不到
ip2hostname: コマンドが見つかりません