ifdown: 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 CentOS possible default full paths are:

/sbin/ifdown - from package: initscripts

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/sbin/ifdown - from package: ifupdown
/sbin/ifdown - from package: netscript-2.4

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

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